Patent Publication Number: US-2021163245-A1

Title: Ultrasonic apparatus, detection apparatus, and printing apparatus

Description:
The present application is based on, and claims priority from JP Application Serial Number 2019-216439, filed Nov. 29, 2019, the disclosure of which is hereby incorporated by reference herein in its all. 
     BACKGROUND 
     1. Technical Field 
     The present disclosure relates to an ultrasonic apparatus, a detection apparatus, and a printing apparatus. 
     2. Related Art 
     In the related art, there is known a detection apparatus that detects an abnormality such as wrinkles in a sheet by using ultrasonic waves (see, for example, JP-A-2002-211797). The detection apparatus described in JP-A-2002-211797 transmits ultrasonic waves to a sheet from an ultrasonic transmission apparatus, and receives the ultrasonic waves that passed through the sheet by an ultrasonic reception apparatus. In this detection apparatus, the drive signal input to the ultrasonic transmission apparatus is compared with the reception signal output from the ultrasonic reception apparatus, and the change in the inclination angle of the sheet caused by the wrinkles of the sheet is detected by using a phase shift. 
     However, the position of the wrinkles occurring on the sheet, the shape of the wrinkles, the size of the wrinkles, and the like vary depending on the material of the sheet, the method of transporting the sheet, the ambient environment such as humidity, and the position of the wrinkles that occur has various patterns. For example, in a transport apparatus that transports a sheet, there are wrinkles and the like that occur only in the upstream of the conveyance and less likely occur in the downstream. In the wrinkle detection apparatus of JP-A-2002-211797 described above, since only one pair of an ultrasonic transmission apparatus and an ultrasonic reception apparatus are provided, for example, it is difficult to detect wrinkles that occur in the upstream of the sheet and are less likely to occur in the downstream of the sheet. Further, when the wrinkles are formed over a wide range, the inclination of the sheet becomes gentle, and it is difficult to detect the wrinkles by using the phase shift from the comparison between the drive signal and the received signal. The above is described with respect to the wrinkles of the sheet, but the same applies the case where an object other than a sheet is used as an object and abnormalities such as unevenness and the like on the object are detected, and even if ultrasonic waves are transmitted/received to/from only one predetermined location of the object, the abnormality of the object may not be detected in some cases. 
     SUMMARY 
     An ultrasonic apparatus according to a first aspect includes a first ultrasonic sensor that transmits a ultrasonic wave to an object and receives the ultrasonic wave reflected by the object, a second ultrasonic sensor that transmits an ultrasonic wave to the object and receives the ultrasonic wave reflected by the object, an error output portion that outputs an error signal when the difference between a first distance between the first ultrasonic sensor and the object calculated based on ultrasonic wave transmission and reception processing using the first ultrasonic sensor and a second distance between the second ultrasonic sensor and the object calculated based on ultrasonic wave transmission and reception processing using the second ultrasonic sensor is equal to or greater than a threshold value. 
     A detection apparatus according to a second aspect includes the ultrasonic apparatus according to the first aspect, and a detector that detects an abnormality of the object based on the error signal output from the ultrasonic apparatus. 
     A printing apparatus according to a third aspect includes a detection apparatus according to the second aspect and a printer that forms an image on the object, and controls printing by the printer based on a detection result of the abnormality by the detector. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view showing a schematic configuration of a printing apparatus according to a first embodiment. 
         FIG. 2  is a block view showing a functional configuration of the printing apparatus according to the first embodiment. 
         FIG. 3  is a schematic view showing a schematic configuration in the vicinity of a platen of the printing apparatus according to the first embodiment. 
         FIG. 4  is a cross-sectional view of an ultrasonic apparatus taken along the line IV-IV of  FIG. 3 . 
         FIG. 5  is a cross-sectional view of the ultrasonic apparatus taken along the line V-V of  FIG. 3 . 
         FIG. 6  is a cross-sectional view showing a schematic configuration of a first ultrasonic sensor of the first embodiment. 
         FIG. 7  is a view showing a schematic configuration of a first protective member of the first embodiment. 
         FIG. 8  is a view showing positions of a transmission and reception surface of a first ultrasonic sensor, a first opening window, a first protective member, and a shape of an ultrasonic beam. 
         FIG. 9  is a view showing positions of a transmission and reception surface of a second ultrasonic sensor, a second opening window, a second protective member, and a shape of an ultrasonic beam. 
         FIG. 10  is a view showing changes in a voltage value of a received signal when a distance from the first ultrasonic sensor to the first protective member is changed in a plurality of patterns in which a disposition angle of the first protective member is changed. 
         FIG. 11  is a view showing measurement results of a magnitude of a received signal when the first ultrasonic sensor receives ultrasonic waves having multiple reflection components, measured by changing the angle of the first protective member. 
         FIG. 12  is a view showing a positional relationship between the first protective member and the first ultrasonic sensor. 
         FIG. 13  is a view showing an example of wrinkles on a sheet detected by a detector. 
         FIG. 14  is a view showing another example of the wrinkles on the sheet detected by the detector. 
         FIG. 15  is a view showing another example of the wrinkles on the sheet detected by the detector. 
         FIG. 16  is a cross-sectional view showing a schematic configuration of an ultrasonic apparatus according to a second embodiment. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     First Embodiment 
     A first embodiment will be described below. 
     Schematic Configuration of Printing Apparatus  100   
       FIG. 1  is a schematic view showing a schematic configuration of a printing apparatus  100  according to a first embodiment.  FIG. 2  is a block view showing a functional configuration of the printing apparatus  100 . The printing apparatus  100  according to the present embodiment is an apparatus that prints an image on a large-sized sheet  1  (object) such as a sign display. As shown in  FIGS. 1 and 2 , the printing apparatus  100  includes a supplier  110 , a transporter  120 , a heater  130 , a carriage  140 , a movement mechanism  150 , and a controller  160  (see  FIG. 2 ). 
     The supplier  110  is a section for supplying the sheet  1 . In the example shown in  FIG. 1 , the supplier  110  is configured to supply the sheet  1  wound around a core material  1 A to the inside of the apparatus. The supplier  110  includes, for example, a core material holder  111  that holds the core material  1 A, and supplies the sheet  1  to the inside of the apparatus by rotating the core material holder  111 . The configuration of the supplier  110  is not limited to the configuration shown in  FIG. 1 . For example, the sheets  1  placed on a tray or the like may be supplied to the inside of the apparatus one by one. In addition, the type of the sheet  1  supplied from the supplier  110  is not particularly limited, and various types of media such as a paper surface, a film, and a woven fabric can be used. 
     The transporter  120  constitutes a transport mechanism and transports the sheet  1  supplied from the supplier  110  along a transport path  10 . In the example shown in  FIG. 1 , the transporter  120  transports the sheet  1  toward the downstream of the transport path  10  by winding the leading end of the sheet  1  supplied from the supplier  110 . In such a configuration, it is possible to reversely transport the sheet  1  from the downstream to the upstream by reversing the winding direction of the transporter  120  and the supply direction of the supplier  110 . The configuration of the transporter  120  is not limited to the configuration shown in  FIG. 1 . For example, the sheet  1  may be transported by rotating a plurality of transport rollers. 
     A platen  11  is provided at a portion of the transport path facing the carriage  140 . The platen  11  corresponds to the disposition portion of the present disclosure, and in the present embodiment, ink is ejected from a printer  141  (see  FIG. 2 ) a on the carriage  140  onto the sheet  1  transported on the platen  11 . In addition, the transport direction (first direction) of the sheet  1  at a position facing the platen  11  in the transport path along which the sheet  1  is transported is defined as a Y direction. 
     The heater  130  includes a first heater  131 , a second heater  132 , and a third heater  133 . The first heater  131  is disposed in the downstream of the platen  11  in the transport path  10  and heats the surface of the sheet  1 . The second heater  132  is provided on the platen  11  and heats the back surface of the sheet  1 . The first heater  131  and the second heater  132  are heaters for drying the ink ejected on the sheet  1 . As shown in  FIG. 1 , the third heater  133  is disposed in the upstream of the platen  11  in the transport path  10  and heats the sheet  1  before being transported to the platen  11 , thereby drying the sheet  1  to suppress the occurrence of wrinkles. 
       FIG. 3  is a schematic view showing a schematic configuration in the vicinity of the platen  11  of the printing apparatus  100 . As shown in  FIG. 3 , the carriage  140  is provided at a position facing the platen  11 . Further, the printing apparatus  100  is provided with a shaft  102  extending in an X direction orthogonal to the transport direction (Y direction) of the sheet  1  at a position facing the platen  11 , and both ends of the shaft  102  are fixed to a frame  101  of the printing apparatus  100 . Part of the carriage  140  is slidably engaged with the shaft  102 , which allows the carriage  140  to move in the X direction. The printer  141  is mounted on the carriage  140 , and an ultrasonic apparatus  200  is attached on the side surface of the carriage  140 . The specific configurations of the printer  141  mounted on the carriage  140  and the ultrasonic apparatus  200  attached on the carriage  140  will be described later. 
     The movement mechanism  150  is an apparatus for moving the carriage  140  in the X direction based on a command from the controller  160 . Although illustration of a specific configuration of the movement mechanism  150  is omitted, for example, a configuration including a timing belt disposed parallel to the shaft  102  and a drive motor for driving the timing belt can be exemplified. In such a configuration, it is possible to move the carriage  140  to a +X side by rotating the drive motor in the normal direction and to move the carriage  140  to a −X side by reversing the drive motor. The configuration of the movement mechanism  150  is not limited to the above, and may be any configuration as long as the movement mechanism  150  allows the carriage  140  to reciprocate in the X direction. 
     Next, the printer  141  mounted on the carriage  140  and the ultrasonic apparatus  200  attached on the carriage  140  will be described. As shown in  FIG. 3 , the carriage  140  of the present embodiment includes the printer  141 . The ultrasonic apparatus  200  is fixed to the side surface of the carriage  140  on the +X side. 
     Configuration of Printer  141   
     The printer  141  has nozzles that individually eject ink in a portion facing the sheet  1  transported to the platen  11 . A plurality of nozzles are provided corresponding to inks of a plurality of colors. For example, a piezo element is disposed in these nozzles, and by driving the piezo element, the ink droplets supplied from the ink tank are ejected from the nozzle. 
     Configuration of Ultrasonic Apparatus  200   
       FIG. 4  is a cross-sectional view of the ultrasonic apparatus  200  taken along the line IV-IV of  FIG. 3 , and  FIG. 5  is a cross-sectional view of the ultrasonic apparatus  200  taken along the line V-V of  FIG. 3 . As shown in  FIGS. 3 and 4 , the ultrasonic apparatus  200  includes a first ultrasonic sensor  210 , a second ultrasonic sensor  220 , a first pedestal portion  231 , a second pedestal portion  232 , and a circuit board  240 , a first protective member  251 , a second protective member  252 , a first holder  261 , a second holder  262 , and a shield member  300 . 
     Configuration of Shield Member  300   
     The shield member  300  is a box-shaped member in which the first ultrasonic sensor  210 , the second ultrasonic sensor  220 , the first pedestal portion  231 , the second pedestal portion  232 , the circuit board  240 , the first protective member  251 , the second protective member  252 , the first holder.  261 , and the second holder  262  are provided. The outer shape of the shield member may be a rectangular parallelepiped, a cylindrical shape, or any other shape. In the present embodiment, an example in which the shield member  300  is formed in a rectangular parallelepiped shape is shown. 
     Specifically, the shield member  300  is made of a conductive material such as metal, and is formed in a container box shape having an opening on the side facing the platen  11 . That is, the shield member  300  includes a rectangular top surface portion  310  disposed on a −Z side, a first side surface portion  321 , a second side surface portion  322 , a third side surface portion  323 , and a fourth side surface portion  324  rising from the edge of the top surface portion  310 , and a rectangular bottom surface portion  330  disposed on a +Z side. The first side surface portion  321  and the second side surface portion  322  are side surfaces parallel to a ZY plane, and among the side surface portions, the first side surface portion  321  is fixed in contact with the +X side surface of the carriage  140 . The third side surface portion  323  and the fourth side surface portion  324  are side surfaces parallel to a ZX plane, the third side surface portion  323  is disposed on the +Y side, and the fourth side surface portion  324  is disposed on the −Y side. Further, the bottom surface portion  330  is provided with a first opening window  331  and a second opening window  332  provided in the Y direction. The first opening window  331  and the second opening window  332  are through holes that communicate with the inside and the outside of the shield member  300 . Such a shield member  300  may be configured by combining a plurality of parts. For example, the shield main body portion including the top surface portion  310 , the first side surface portion  321 , the second side surface portion  322 , the third side surface portion  323 , and the fourth side surface portion  324 , and the bottom surface portion  330  may be detachably provided in the shield main body portion. Further, the shield main body portion may be formed by combining a first body including the first side surface portion  321  and a second body including the second side surface portion  322 . 
     In the shield member  300  of the present embodiment, as shown in  FIGS. 4 and 5 , the circuit board  240  is disposed on the side of the first side surface portion  321  in the internal space of the shield member  300 , the first ultrasonic sensor  210  is disposed facing the first opening window  331 , and the second ultrasonic sensor  220  is disposed facing the second opening window  332 . For example, as shown in  FIGS. 4 and 5 , the first side surface portion  321  is provided with a first fixing portion  301 , and the circuit board  240  is fixed to the first fixing portion  301 . Further, a second fixing portion  302  and a third fixing portion  303  are provided on the first side surface portion  321  and the second side surface portion  322 . The first pedestal portion  231  to which the first ultrasonic sensor  210  is fixed is fixed to the second fixing portion  302 , and the second pedestal portion  232  to which the second ultrasonic sensor  220  is fixed is fixed to the third fixing portion  303 . In addition, the top surface portion  310  is provided with a wiring hole  311  through which a coupling wire that couples the circuit board  240  and the controller  160  is inserted. 
     Further, the shield member  300  includes a holder holding portion  304  between the second fixing portion  302  and the third fixing portion  303 , and the bottom surface portion  330 . The holder holding portion  304  includes a first engaging portion  304 A that detachably engages the first holder  261  between the first ultrasonic sensor  210  and the first opening window  331 , and a second engaging portion  304 B that detachably engages the second holder  262  between the second ultrasonic sensor  220  and the second opening window  332 . Then, the first holder  261  to which the first protective member  251  is fixed is held in the first engaging portion  304 A, and the second holder  262  to which the second protective member  252  is fixed is held in the second engaging portion  304 B. As a result, the first protective member  251  and the second protective member  252  are provided inside the shield member  300 . Hereinafter, each configuration provided inside the shield member  300  will be described in detail. 
     Configuration of Ultrasonic Sensors  210  and  220   
     The first ultrasonic sensor  210  and the second ultrasonic sensor  220  are sensors that transmit ultrasonic waves toward the sheet  1  and receive ultrasonic waves reflected by the sheet  1 .  FIG. 6  is a cross-sectional view showing a schematic configuration of the first ultrasonic sensor  210 . Since the first ultrasonic sensor  210  and the second ultrasonic sensor  220  have the same configuration, the configuration of the first ultrasonic sensor  210  will be described here, and the configuration of the second ultrasonic sensor  220  will be omitted. 
     As shown in  FIG. 6 , the first ultrasonic sensor  210  includes an element substrate  41  and a piezoelectric element  42 . The element substrate  41  includes a substrate body portion  411  and a vibrating plate  412  provided on one surface side of the substrate body portion  411 . Here, in the following description, the substrate thickness direction of the element substrate  41  is a Z direction. The Z direction is a transmission and reception direction in which ultrasonic waves are transmitted from the first ultrasonic sensor  210 , and is a direction intersecting the X direction and the Y direction. The substrate body portion  411  is a substrate that supports the vibrating plate  412 , and is made of a semiconductor substrate such as Si. The substrate body portion  411  is provided with an opening portion  411 A that penetrates the substrate body portion  411  along the Z direction. 
     The vibrating plate  412  is made of SiO 2 , a stacked body of SiO 2  and ZrO 2 , or the like, and is provided on the −Z side of the substrate body portion  411 . The vibrating plate  412  is supported by a partition wall  411 B of the substrate body portion  411  that constitutes the opening  411 A, and closes the −Z side of the opening  411 A. A portion of the vibrating plate  412  that overlaps the opening portion  411 A when viewed from the Z direction constitutes a vibrating portion  412 A. 
     The piezoelectric element  42  is provided on the vibrating plate  412  and at a position overlapping each vibrating portion  412 A when viewed from the Z direction. As shown in  FIG. 6 , the piezoelectric element  42  is configured by sequentially stacking a first electrode  421 , a piezoelectric film  422 , and a second electrode  423  on the vibrating plate  412 . 
     Here, one vibrating portion  412 A and the piezoelectric element  42  provided on the vibrating portion  412 A constitute one ultrasonic transducer Tr. Although illustration is omitted, in the present embodiment, the first ultrasonic sensor  210  is configured by arranging such ultrasonic transducers Tr in a two-dimensional array structure. 
     In the first ultrasonic sensor  210 , the piezoelectric film  422  expands and contracts when a pulse wave voltage of a predetermined frequency is applied between the first electrode  421  and the second electrode  423  of each ultrasonic transducer Tr. As a result, the vibrating portion  412 A vibrates at a frequency according to the opening width of the opening portion  411 A and the like, and ultrasonic waves are transmitted from the vibrating portion  412 A toward the +Z side. Further, when the ultrasonic waves are input from the opening  411 A, the vibrating portion  412 A vibrates, and a potential difference is generated between the first electrode  421  side and the second electrode  423  side of the piezoelectric film  422 . As a result, the first ultrasonic sensor  210  outputs a reception signal according to the potential difference generated in the piezoelectric film  422 . In such a configuration, the +Z side surface of the element substrate  41  serves as an ultrasonic wave transmission and reception surface  211  of the first ultrasonic sensor  210 . 
     As described above, the second ultrasonic sensor  220  has the same configuration as the first ultrasonic sensor  210 . That is, the second ultrasonic sensor  220  is configured to include the element substrate  41  and the piezoelectric element  42 , and the +Z side surface of the element substrate  41  is used as the ultrasonic wave transmission and reception surface  221  of the second ultrasonic sensor  220  to perform ultrasonic wave transmission and reception processing. 
     Configuration of Pedestal Portions  231  and  232   
     The first pedestal portion  231  has a flat surface facing the first opening window  331 , and the first ultrasonic sensor  210  is fixed to the flat surface. Similarly, the surface of the second pedestal portion  232  facing the second opening window  332  is formed into a flat surface, and the second ultrasonic sensor  220  is fixed to the flat surface. As described above, the first pedestal portion  231  is fixed to the second fixing portion  302  of the shield member  300 , and the second pedestal portion  232  is fixed to the third fixing portion  303  of the shield member  300 . In the present embodiment, an example of fixing the first pedestal portion  231  and the second pedestal portion  232  to the second fixing portion  302  and the third fixing portion  303  provided on the first side surface portion  321  and the second side surface portion  322  of the shield member is shown, but is not limited to thereto. For example, the second fixing portion  302  and the third fixing portion  303  may be fixed to the circuit board  240  fixed to the shield member  300 . 
     Configuration of Circuit Board  240   
     The circuit board  240  is disposed parallel or substantially parallel to the first side surface portion  321  and the second side surface portion  322 . More specifically, the circuit board  240  is disposed such that the projected area of the circuit board  240  when projected onto a YZ plane is 70% or more of the area of the circuit board  240 . The area of the circuit board  240  is the area of one surface orthogonal to the board thickness of the circuit board  240 . That is, the angle formed between the board surface of the circuit board  240  and the YZ plane is 0° or more and 45° or less. With such a configuration, for example, the ultrasonic apparatus  200  can be downsized as compared with the case where the circuit board  240  is disposed parallel to the top surface portion  310  and the bottom surface portion  330 . That is, in the present embodiment, the ultrasonic apparatus  200  on the +X side of the carriage  140  is fixed, and the carriage  140  can be moved in the X direction by the movement mechanism  150 . In such a configuration, when the circuit board  240  is disposed parallel to an XY plane, the width of the ultrasonic apparatus  200  in the X direction increases, and the moving range of the carriage  140  decreases by the increased dimension, or the printing apparatus  100  may be increased in size. On the other hand, in the present embodiment, by disposing the circuit board  240  to be parallel to the YZ plane, it is possible to suppress an increase in the width of the ultrasonic apparatus  200  in the X direction. Further, the circuit board  240  is disposed closer to the first side surface portion  321  than the midpoint between the first side surface portion  321  and the second side surface portion  322 . That is, the circuit board  240  is disposed between the first side surface portion  321  and the midpoint between the first side surface portion  321  and the second side surface portion  322 . 
     This circuit board  240  is coupled to the first ultrasonic sensor  210  and the second ultrasonic sensor  220 , and includes a first control circuit  241 , a second control circuit  242 , and a determination circuit  243  as shown in  FIG. 2 . The first control circuit  241  outputs, to the first ultrasonic sensor  210 , a drive signal for transmitting ultrasonic waves from the first ultrasonic sensor  210 . In addition, the first control circuit  241  receives a reception signal output from the first ultrasonic sensor  210  when the first ultrasonic sensor  210  receives an ultrasonic wave, and performs signal processing such as amplification processing. Further, the first control circuit  241  calculates a first distance, which is the distance between the first ultrasonic sensor  210  and the sheet  1 , based on the time from the transmission timing of ultrasonic waves to the reception timing of ultrasonic waves in the first ultrasonic sensor  210 . 
     The second control circuit  242  outputs, to the second ultrasonic sensor  220 , a drive signal for transmitting ultrasonic waves from the second ultrasonic sensor  220 . In addition, the second control circuit  242  receives a reception signal output from the second ultrasonic sensor  220  when the second ultrasonic sensor  220  receives an ultrasonic wave, and performs signal processing such as amplification processing. Further, the second control circuit  242  calculates a second distance, which is the distance between the second ultrasonic sensor  220  and the sheet  1 , based on the time from the transmission timing of ultrasonic waves to the reception timing of ultrasonic waves in the second ultrasonic sensor  220 . 
     The determination circuit  243  corresponds to an error output portion according to the present disclosure, and determines whether the difference between the first distance calculated by the first control circuit  241  and the second distance calculated by the second control circuit is within a predetermined threshold value. Then, the determination circuit  243  outputs an error signal to the controller  160  when the difference between the first distance and the second distance exceeds the threshold value. That is, when the difference between the first distance and the second distance is large, there is a possibility that an abnormality such as wrinkles has occurred in the sheet  1 , and therefore an error signal indicating this possibility is output to the controller  160 . 
     Configuration of Protective Members  251  and  252  and Holders  261  and  262   
       FIG. 7  is a view showing a schematic configuration of the first protective member  251 . Since the second protective member  252  has the same configuration as the first protective member  251 , the description thereof will be omitted here. In  FIG. 7 , an Xm direction and a Ym direction are directions intersecting with the Z direction, and the normal line of an XmYm plane is inclined at a predetermined angle θ with respect to the Z direction. In the following description, the normal line of the XmYm plane may be referred to as the normal line of the first protective member  251  or the normal line of the second protective member  252 . As shown in  FIG. 7 , a plurality of wire rods  253  having the Xm direction as a line direction are disposed along the Ym direction, and a plurality of wire rods  253  having the Ym direction as a line direction are disposed along the Xm direction, whereby the first protective member  251  is a filter configured in a mesh shape. Although  FIG. 7  shows an example in which the Xm direction and the Ym direction intersect at 90°, the present disclosure is not limited thereto, and the angle formed by the Xm direction and the Ym direction may be an angle other than 90°. As the material of the wire rod  253 , a metal material such as copper, iron, brass, or SUS, an alloy material, a synthetic resin such as nylon or polyester, or the like can be used. In particular, it is preferable to use a material having conductivity, and in this case, resistance to static electricity and electromagnetic waves can be obtained. 
     Further, a wire diameter W 1  of the wire rod  253  is preferably less than the wavelength of ultrasonic waves. This suppresses the disadvantage that the ultrasonic waves are diffusely reflected by the wire rod  253 . In such a first protective member  251 , a void  254  surrounded by a pair of wire rods  253  adjacent to each other in the Xm direction and a pair of wire rods  253  adjacent to each other in the Ym direction is formed, and this void corresponds to a first hole portion according to the present disclosure through which ultrasonic waves pass. Similarly, in the second protective member  252 , the void  254  surrounded by a pair of wire rods  253  adjacent to each other in the Xm direction and a pair of wire rods  253  adjacent to each other in the Ym direction is formed, and this void corresponds to a second hole portion according to the present disclosure through which ultrasonic waves pass. In the present embodiment, in order to suppress the adhesion of foreign matter such as ink droplets and paper dust to the first ultrasonic sensor  210  and the second ultrasonic sensor  220 , it is preferable that the width of the void  254 , that is, the distance (open W2) between the adjacent wire rods  253  is set to 1 mm or less. 
     The distance between the center axes of the wire rods  253  is defined as a pitch W3, and a porosity S is defined by the following (1). 
         S= 100×( W   2   /W   3 ) 2   (1)
 
     In the present embodiment, a porosity S is preferably 20% or more. When the distance to the sheet  1  is measured by the first ultrasonic sensor  210  and the second ultrasonic sensor  220 , the ultrasonic waves transmitted from the first ultrasonic sensor  210  and the second ultrasonic sensor  220  reach the sheet  1 , and the ultrasonic waves reflected by the sheet  1  need to be received by the first ultrasonic sensor  210  and the second ultrasonic sensor  220  again. In this case, when the sound pressure of the received ultrasonic wave decreases, the reception sensitivity of ultrasonic waves decreases, and the reception timing cannot be properly determined. Therefore, in order to suppress the decrease in the sound pressure of ultrasonic waves, it is preferable that the acoustic transmittance of the first protective member  251  and the second protective member  252  be 50% or more. Here, when the porosity S is less than 20%, the acoustic transmittance is less than 50%, and the reception sensitivity decreases. On the other hand, when the porosity S is 20% or more, the acoustic transmittance becomes 50% or more, and it is possible to suppress an excessive decrease in the sound pressure of the received ultrasonic waves. 
     The first holder  261  is a member for holding the first protective member  251 , and is attached to the first engaging portion  304 A provided on the holder holding portion  304  of the shield member  300 . The second holder  262  is a member that holds the second protective member  252 , and is attached to the second engaging portion  304 B provided on the holder holding portion  304  of the shield member  300 . 
     As shown in  FIGS. 4 and 5 , the first holder  261  has a first holding surface  263  for holding the first protective member  251 . The first holding surface  263  is a plane inclined with respect to the Z direction, and the first protective member  251  is fixed along the first holding surface  263 . Thereby, in the first protective member  251 , the surface (first protective surface  251 A) facing the first ultrasonic sensor  210  is inclined with respect to the transmission and reception surface  211  of the first ultrasonic sensor  210 . That is, the first protective surface  251 A is inclined at an angle θ with respect to the Z direction. In addition, the first holding surface  263  is provided with a first passage hole  261 A through which ultrasonic waves pass. 
     The first holding surface  263  provided with the first protective member  251  is inclined so that the distance from the transmission and reception surface  211  of the first ultrasonic sensor  210  increases toward the +X side, for example. As a result, the first protective surface  251 A of the first protective member  251  provided on the first holding surface  263  is also inclined so that the distance between the first protective surface  251 A and the transmission and reception surface  211  increases toward the +X side. 
     The same applies to the second holder  262 , and the second holder  262  has a second holding surface  264  for holding the second protective member  252  as shown in  FIG. 4 . The second holding surface  264  is provided with a second passage hole  262 A that allows ultrasonic waves to pass therethrough. Although not shown in  FIG. 5 , the second holding surface  264  of the second holder  262  is, for example, a plane inclined with respect to the Z direction, like the first holding surface  263 , and the distance from the transmission and reception surface  221  of the second ultrasonic sensor  220  is inclined so as to increase toward the +X side. As a result, the surface (second protective surface  252 A) of the second protective member  252  provided on the second holding surface  264  of the second holder  262  facing the second ultrasonic sensor  220  also inclines so that the distance between the second protective surface  252 A and the transmission and reception surface  221  increases toward the +X side. 
     Positional Relationship Between Transmission and Reception Surfaces  211  and  221  and Opening Windows  331  and  332   
       FIG. 8  is a view showing the positions of the transmission and reception surface  211  of the first ultrasonic sensor  210 , the first opening window  331 , and the first protective member  251 , and the shape of the ultrasonic beam, and  FIG. 9  is a view showing the positions of the transmission and reception surface  221 , the second opening window  332 , and the second protective member  252  of the second ultrasonic sensor  220 , and the shape of the ultrasonic beam. As shown in  FIG. 8 , in the ultrasonic apparatus  200  of the present embodiment, a distance Z S1  between the transmission and reception surface  211  of the first ultrasonic sensor  210  and the first opening window  331  of the shield member  300  is shorter than a near-field limit distance Z N  of the ultrasonic waves transmitted from the first ultrasonic sensor  210 . Therefore, the first protective member  251  is provided within the near-field limit distance Z N . Further, it is preferable that the distance from the first ultrasonic sensor  210  to the platen  11  is in the vicinity of the near-field limit distance Z N . As a result, ultrasonic waves with high sound pressure can be applied to the sheet  1  disposed on the platen  11 , and an S/N ratio of the received signal can be improved. 
     Further, as shown in  FIG. 8 , a width S TR1  of the transmission and reception surface  211  of the first ultrasonic sensor  210 , an opening size S S1  of the first opening window  331 , and an opening dimension S m1  when the first passage hole  261 A of the first holder  261  is projected on the XY plane satisfy the relationship of S TR1 ≤S m1 ≤S S1 , and more preferably S TR1 &lt;S m1 &lt;S S1 . 
     The same applies to the second ultrasonic sensor  220 , the second opening window  332 , and the second passage hole  262 A, as shown in  FIG. 9 . That is, the distance Z S2  between the transmission and reception surface  221  of the second ultrasonic sensor  220  and the second opening window  332  is shorter than the near-field limit distance Z N  of the ultrasonic waves transmitted from the second ultrasonic sensor  220 , and the second protective member  252  is provided within the near-field limit distance Z N . In the present embodiment, Z S1 =Z S2 , but the distance Z S1  between the first ultrasonic sensor  210  and the first opening window  331  and the distance Z S2  between the second ultrasonic sensor  220  and the second opening window  332  may be a different distance. Further, it is preferable that the distance between the second ultrasonic sensor  220  and the platen  11  is in the vicinity of the near-field limit distance Z N . Further, the width S TR2  of the transmission and reception surface  221  of the second ultrasonic sensor  220 , the opening size S S2  of the second opening window  332 , and the opening size S m2  when the second passage hole is projected on the XY plane satisfy the relationship of S TR2 ≤S m2 ≤S S2 , and more preferably S TR2 &lt;S m2 &lt;S S2 . 
     As described above, in the present embodiment, the distance Z S1  from the first ultrasonic sensor  210  to the first opening window  331  and the distance Z S2  from the second ultrasonic sensor  220  to the second opening window  332  are shorter than the near-field limit distance Z N . Within this near-field limit distance Z N , the beam diameter of the ultrasonic waves is approximately the same as the widths S TR1  and S TR2  of the transmission and reception surfaces  211  and  221  as shown in  FIGS. 8 and 9 . Therefore, as described above, by setting the relationship of S TR1 ≤S m1 ≤S S1  and S TR2 ≤S m2 ≤S S2 , the reflection of ultrasonic waves on the first holder  261  and the second holder  262  and the reflection on the bottom surface portion  330  can be suppressed. Further, as the shield member  300 , in the configuration in which the bottom surface portion  330  is attachable to and detachable from the shield main body portion including the top surface portion  310  and the side surface portions  321  to  324 , after the ultrasonic sensors  210  and  220 , the circuit board  240 , and the like are provided inside the shield main body portion, the bottom surface portion  330  is attached to the shield main body portion. With such a configuration, the first opening window  331  and the second opening window  332  are easily affected by the positional balance during assembly of the ultrasonic apparatus  200 , and it is difficult to adjust the alignment when the bottom surface portion  330  is attached. On the other hand, by setting S TR1 &lt;S m1 &lt;S S1  and S TR2 &lt;S m2 &lt;S S2 , it is easy to adjust the alignment when attaching the bottom surface portion  330  to the shield body, and the positions of the first opening window  331  and the second opening window  332  can be suppressed from deviating from the position where the ultrasonic beam is formed. 
     Configuration for Suppressing Multiple Reflection of Ultrasonic Waves 
     Next, the influence of multiple reflection by the first protective member  251  will be described. Since the second protective member  252  has the same configuration as the first protective member  251 , the description here will be omitted or simplified.  FIG. 10  is a view showing changes in the voltage value of the received signal when the distance Z m1  from the transmission and reception surface  211  to the first protective member  251  is changed, in a plurality of patterns in which the disposition angle of the first protective member  251  is changed. In  FIG. 10 , the angle formed between the normal line of the first protective surface  251 A and the Z direction is changed into three patterns of 0°, 10°, and 20°, and in each case, the distance Z m1  between the first ultrasonic sensor  210  and the first protective member  251  is changed between 3 mm and 10 mm. In  FIG. 10 , a signal A1 is a received signal when the normal line of the first protective surface  251 A and the Z direction are parallel to each other. A signal A2 is a received signal when the first angle θ formed between the normal line of the first protective surface  251 A and the Z direction is 10°. A signal A3 is a received signal when the first angle θ formed between the normal line of the first protective surface  251 A and the Z direction is 20°. The display of the signal A3 when the distance Z m1  is changed from 3 mm to 5 mm is omitted in consideration of the visibility of the signal A2, but the same waveform as after 5 mm is obtained. 
     When the normal line of the first protective surface  251 A and the Z direction are parallel to each other, if the distance Z m1  is changed as shown by the signal A1 in  FIG. 10 , the voltage value (received voltage) of the received signal fluctuates greatly. That is, when the relationship between the distance Z m1  and a wavelength λ of the ultrasonic wave is Z m =m×λ/2 (where m is an integer), the ultrasonic waves due to the multiple reflection components strengthen each other, and when the relationship between a distance L 1  and the wavelength λ of the ultrasonic wave is the distance Z m1 ={(2 m+1)/4}×λ, the ultrasonic waves due to the multiple reflection components weaken each other. As described above, when the variation in the voltage value of the received signal becomes large when the distance Z m1  is changed, it means that the multiple reflection component of the ultrasonic wave is received by the first ultrasonic sensor  210 . In such a case, it is difficult to accurately detect the reception timing of the ultrasonic wave reflected by the sheet  1  due to the noise component of the ultrasonic wave that is multiply reflected. 
     On the other hand, when the first angle θ is set to 10° or more like the signals A2 and A3, the variation of the received signal when the distance Z m1  is changed becomes small. This means that the multiple reflection component of the ultrasonic wave received by the first ultrasonic sensor  210  is reduced. That is, by increasing the first angle θ, noise due to multiple reflection components can be suppressed, and the reception timing of the ultrasonic waves reflected by the sheet  1  can be accurately detected. 
       FIG. 11  shows the measurement result of the magnitude of the received signal when the first ultrasonic sensor  210  receives the ultrasonic wave of the multiple reflection component, which is measured by changing the first angle θ of the first protective member  251 . As shown in  FIG. 11 , when the first angle θ formed between the normal line of the first protective surface  251 A and the Z direction is increased, the voltage value of the received signal decreases. 
     In order to suppress the deterioration of the detection accuracy of the reception timing due to the multiple reflection, it is preferable to set the inclination angle of the first protective member  251  so that at least the voltage value of the received signal due to the multiple reflection becomes equal to or less than the half value of the received voltage when the first angle θ is 0°. In this case, as shown in  FIG. 11 , by setting the first angle θ to 5° or more, the voltage value of the received signal can be equal to or less than the half value of the received signal when θ=0°, regardless of the distance Z m1 . 
     Considering the strengthening or weakening of the ultrasonic waves due to the multiple reflection components described above, it is more preferable that the first angle θ of the first protective member  251  is 10° or more.  FIG. 12  is a view showing a positional relationship between the first protective member  251  and the first ultrasonic sensor  210 . In  FIG. 12 , the distance Z m1  between the first ultrasonic sensor  210  and the first protective member  251  is the distance between the center point of the transmission and reception surface  211  of the first ultrasonic sensor  210  and the center point of the first protective member  251 . When the ultrasonic wave transmitted from the center point of the transmission and reception surface  211  in the Z direction is reflected by the first protective member  251 , the reflected ultrasonic wave is input at a position separated from the center point of the transmission and reception surface  211  by a distance L d =Z m1 ·tan(2θ) in the same plane as the transmission and reception surface  211 . 
     Therefore, in order to prevent the ultrasonic waves reflected by the first protective member  251  from being received by the first ultrasonic sensor  210 , it is preferable to set the position and the inclination angle of the first protective member  251  so that the relationship between the width S TR1  of the transmission and reception surface  211  and the distance L d  is S TR1 /2&lt;L d . That is, it is preferable that the positional relationship between the width S TR1  of the transmission and reception surface  211  of the first ultrasonic sensor  210  and the first protective member  251  satisfies S TR1 &lt;2Z m1 ·tan(2θ). It is more preferable to satisfy the positional relationship between the width S TR1  of the transmission and reception surface  211  of the first ultrasonic sensor  210  and the first protective member  251  satisfies S TR1 &lt;L d , that is, S TR1 &lt;Z m1 ·tan(2θ). In this case, it is possible to further suppress the disadvantage that the ultrasonic wave transmitted from the first ultrasonic sensor  210  is reflected by the first protective member  251  and enters the transmission and reception surface  211 . 
     The above relationship also holds for the second ultrasonic sensor  220  and the second protective member  252 . Therefore, it is preferable that the positional relationship between the width S TR2  of the transmission and reception surface  221  of the second ultrasonic sensor  220  and the second protective member  252  satisfies S TR2 &lt;Z m2 ·tan(2θ), and it is more preferable to satisfy S TR2 &lt;Z m2 ·tan(2θ). 
     Further, in the embodiment, as shown in  FIG. 5 , the first protective surface  251 A to which the ultrasonic wave from the first ultrasonic sensor  210  is input is inclined so as to face the second side surface portion  322 . That is, assuming that distance between any first point on the first protective surface  251 A and the transmission and reception surface  211  is L1, and the distance between the transmission and reception surface  211  and a second point on the +X side farther from the circuit board  240  than the first point is L2, L1&lt;L2 is satisfied. In other words, the first protective member  251  is inclined so that the ultrasonic wave transmitted from the first ultrasonic sensor  210  in the Z direction is reflected by the first protective surface  251 A toward the second side surface portion  322 . 
     Although not shown in  FIG. 5 , the second protective member  252  is also the same, and the second protective surface  252 A to which the ultrasonic wave from the second ultrasonic sensor  220  is input is inclined so as to face the second side surface portion  322 . That is, assuming that the distance between any third point on the second protective surface  252 A and the transmission and reception surface  221  is L3, and the distance between a fourth point on the +X side farther from the circuit board  240  than the third point and the transmission and reception surface  221  is L4, L3&lt;L4 is satisfied. In other words, the second protective member  252  is inclined so that the ultrasonic wave transmitted from the second ultrasonic sensor  220  in the Z direction is reflected by the second protective surface  252 A toward the second side surface portion  322 . 
     As a result, it is possible to suppress the disadvantage that the ultrasonic wave transmitted from the first ultrasonic sensor  210  and reflected by the first protective member  251  is reflected by the circuit board  240 . That is, in the present embodiment, as described above, as shown in  FIG. 5 , the first ultrasonic sensor  210  and the second ultrasonic sensor  220  are provided at a midpoint of the shield inner space from the first side surface portion  321  to the second side surface portion  322 . On the other hand, the circuit board  240  is disposed so as to be closer to the first side surface portion  321  than the midpoint of the shield inner space from the first side surface portion  321  to the second side surface portion  322 , and to be parallel to the first side surface portion  321  and the second side surface portion  322 . Therefore, the distance between the first ultrasonic sensor  210  and the second ultrasonic sensor  220  and the second side surface portion  322  is greater than the distance between the first ultrasonic sensor  210  and the second ultrasonic sensor  220  and the circuit board  240 . 
     Here, the case where the ultrasonic waves transmitted from the first ultrasonic sensor  210  and the second ultrasonic sensor  220  are reflected toward the circuit board  240  by the first protective member  251  and the second protective member  252  and the case where the ultrasonic waves are reflected on the second side surface portion  322  side will be compared. The circuit board  240  is disposed closer to the first ultrasonic sensor  210  and the second ultrasonic sensor  220  than the second side surface portion  322 . Therefore, in the former case, when the ultrasonic waves are reflected toward the circuit board  240 , a large amount of the ultrasonic waves re-reflected by the circuit board  240  are input to the first ultrasonic sensor  210  and the second ultrasonic sensor  220 , and the received signal has a lot of noise. On the other hand, in the latter, even when the ultrasonic waves are re-reflected by the second side surface portion  322 , the amount of ultrasonic waves input to the first ultrasonic sensor  210  and the second ultrasonic sensor  220  is smaller than that of the former, and the noise included in the received signal is also small. 
     Configuration of Controller  160   
     As shown in  FIG. 2 , the controller  160  includes an arithmetic portion  161  configured by a CPU (Central Processing Unit) and the like, and a storage portion  162  configured by a recording circuit such as a memory. The controller  160  is coupled to the supplier  110 , the transporter  120 , the heater  130 , the printer  141 , the movement mechanism  150 , and the ultrasonic apparatus  200 , and controls the overall operation of the printing apparatus  100 . The controller  160  is coupled to an interface portion (not shown), and is coupled to an external apparatus such as a personal computer via the interface portion. Then, the controller  160  receives the image data input from the external apparatus, controls each portion of the printing apparatus  100 , and forms an image on the sheet  1  based on the image data. 
     The storage portion  162  records various data for controlling the printing apparatus  100  and various programs. The arithmetic portion  161  functions as a first controller  163 , a detector  164 , a second controller  165 , and the like by reading and executing various programs stored in the storage portion  162 , as shown in  FIG. 2 . 
     The first controller  163  controls the supplier  110  and the transporter  120  to transport the sheet  1  so that the predetermined position of the sheet  1  is located on the platen  11 . The first controller  163  also controls the movement mechanism  150  to move the carriage  140  and the ultrasonic apparatus  200  to a predetermined position on the platen  11 . 
     The detector  164  commands the ultrasonic apparatus  200  to perform ultrasonic measurement, and detects the occurrence of wrinkles based on the error signal from the ultrasonic apparatus  200 . The detection apparatus according to the present disclosure is configured by the ultrasonic apparatus  200  and the function as the detector  164  of the arithmetic portion  161 . Here, the method for detecting wrinkles in the present embodiment will be described in detail. As shown in  FIG. 2 , in the ultrasonic apparatus  200  of the present embodiment, the first ultrasonic sensor  210  is disposed in the downstream and the second ultrasonic sensor  220  is disposed in the upstream along the Y direction, which is the transport direction of the sheet  1 . Further, since the ultrasonic apparatus  200  is fixed to the carriage  140 , the ultrasonic apparatus  200  can also be moved in the X direction by moving the carriage  140  by the movement mechanism  150 . Therefore, the first ultrasonic sensor  210  can perform ultrasonic wave transmission and reception processing at each position along the X direction in the downstream in the transport direction. Further, the second ultrasonic sensor  220  can perform ultrasonic wave transmission and reception processing at each position along the X direction in the upstream in the transport direction. Accordingly, the first control circuit  241  can calculate the first distance from the first ultrasonic sensor  210  to the sheet  1  at the downstream in the transport direction and at each position along the X direction, and the second control circuit  242  can calculate the second distance from the second ultrasonic sensor  220  to the sheet  1  at the upstream in the transport direction and at each position along the X direction. Further, the determination circuit  243  calculates the difference between the first distance and the second distance at each position in the X direction, and outputs an error signal when the difference exceeds a threshold value. 
     Therefore, when no error signal is input from the ultrasonic apparatus  200 , the detector  164  can determine that wrinkles are not detected in the sheet  1  at each position in the X direction, and can detect wrinkles in the sheet  1  in any position in the X direction when an error signal is input from the ultrasonic apparatus  200 . 
       FIGS. 13 to 15  are views showing examples of wrinkles in the sheet  1  detected by the detector  164 .  FIGS. 13 to 15  show the amount wrinkles occurring in the sheet  1  with respect to the six locations C1(X1, Y1), C2(X2, Y1), C3(X3, Y1), C4(X1, Y2), C5(X2, Y2), and C6(X3, Y2), and black circles indicate locations with many wrinkles, white circles indicate locations with medium wrinkles, and double circles indicate locations with few wrinkles. For example, in the example of  FIG. 13 , this is a wrinkle that occurs when the ±X side edge of the sheet  1  is inclined with respect to the Y direction, which is the transport direction. Originally, the ±X edge of the sheet  1  is transported in a state of being parallel to the Y direction as shown in  FIG. 13 . However, as shown in  FIG. 13 , when the ±X side edge of the sheet  1  is inclined with respect to the Y direction due to impact or the like, and the sheet  1  is transported in the Y direction as it is, wrinkles (serpentine wrinkles) occur on the sheet  1  at the positions of C5 and C6 in the upstream in the Y direction and the positions of C3 in the downstream. In this case, for example, the occurrence of wrinkles cannot be detected only at the position of C1. On the other hand, in the present embodiment, the detector  164  can detect wrinkles because an error signal is output from the ultrasonic apparatus  200  when the ultrasonic apparatus  200  is moved to the position of X2. 
     The wrinkles on the sheet  1  may change depending on the type of the sheet  1 . For example,  FIG. 14  shows an example of wrinkles when a wallpaper for a sign display is used as the sheet  1 , and in this example, wrinkles occur only at the position of C5. Therefore, the detector  164  can detect wrinkles because the ultrasonic apparatus  200  outputs an error signal when the ultrasonic apparatus  200  is moved to the position of X2. Further, the example of  FIG. 15  is an example of wrinkles when a woven fabric for a banner, which is also used as a banner, is used as the sheet, and many wrinkles occur at the position of C3, and medium wrinkles occur at the position of C6. Therefore, the detector  164  can detect wrinkles because the ultrasonic apparatus  200  outputs an error signal when the ultrasonic apparatus  200  is moved to the position of X3. 
       FIGS. 13 to 15  show examples of serpentine wrinkles, but the wrinkles that occur on the sheet  1  include, for example, wrinkles that occur when the sheet  1  is left in a high humidity environment, swelling wrinkles that occur when the sheet  1  made of a material having high rigidity and shrinkability is used, and wrinkles caused by the sticking of the sheet  1  due to static electricity, wrinkles that occur when the end surface of the roll paper floats up when roll paper is used as the sheet  1 , and the like. Since it is a wrinkle that causes unevenness on the surface of the sheet  1 , the wrinkles left alone change the first distance and the second distance due to unevenness at a plurality of locations along the X direction, and an error signal is output. Since wrinkles in which part of the sheet rises occur, an error signal is output from the ultrasonic apparatus  200  in the swelling wrinkles at the raised location. When sticking occurs due to static electricity, the ultrasonic apparatus  200  outputs an error signal at the position where the sticking occurs. The wrinkles occurred by the floating of the end surface of the roll paper cause an error signal to be output from the ultrasonic apparatus  200  at the location where the roll paper floats. Therefore, in the present embodiment, the detector  164  can suitably detect wrinkles with respect to any of the above wrinkles. 
     The second controller  165  controls the printer  141  to form an image on the sheet  1  when no wrinkles are detected by the detector  164 . Specifically, the second controller  165  forms an image on the sheet  1  in cooperation with the first controller  163 . That is, in the printing apparatus  100 , when image data is input from an external apparatus or the like, the first controller  163  transports the sheet  1  so that the image formation position of the sheet  1  is located on the platen  11  based on the image data and moves the carriage  140  in the X direction. Then, when the carriage is moved to the position based on the image data, the second controller  165  ejects ink of the color based on the image data to the image formation position to form dots. By repeating the above processing, the first controller  163  and the second controller  165  form an image on the sheet  1 . Further, the second controller  165  suspends the image forming processing when the detector  164  detects wrinkles during the formation of the image. This suppresses the disadvantage of wasted ink. 
     Operational Effects of Present Embodiment 
     The printing apparatus  100  of the present embodiment includes an ultrasonic apparatus  200 . The ultrasonic apparatus  200  includes a first ultrasonic sensor  210 , a second ultrasonic sensor  220 , and a circuit board  240 . The first ultrasonic sensor  210  and the second ultrasonic sensor  220  transmit ultrasonic waves to the sheet  1 , which is an object, and receive ultrasonic waves reflected by the sheet  1 . The circuit board  240  includes a first control circuit  241 , a second control circuit  242 , and a determination circuit  243 . The first control circuit  241  calculates the first distance between the first ultrasonic sensor  210  and the sheet  1  based on the time from the transmission timing of the ultrasonic waves obtained by the transmission and reception processing of the ultrasonic waves using the first ultrasonic sensor  210  to the reception timing, and the speed of sound. The second control circuit  242  calculates the second distance between the second ultrasonic sensor  220  and the sheet  1  based on the time from the transmission timing of the ultrasonic waves obtained by the transmission and reception processing of the ultrasonic waves using the second ultrasonic sensor  220  to the reception timing, and the speed of sound. The determination circuit  243  determines whether the difference between the first distance and the second distance is equal to or greater than a threshold value, and outputs an error signal when the difference is equal to or greater than the threshold value. 
     With such a configuration, the ultrasonic apparatus  200  calculates the distance from the sheet  1  at a plurality of locations on the sheet  1 , and outputs an error signal when the first distance and the second distance have a difference equal to or greater than the threshold value based on the distances calculated at the plurality of locations. Therefore, the wrinkles can be determined with higher accuracy than in the case where the presence or absence of wrinkles is determined by the ultrasonic wave transmission and reception processing for a single location on the sheet  1 . 
     The ultrasonic apparatus  200  of the present embodiment further includes the conductive shield member  300  in which the first ultrasonic sensor  210  and the second ultrasonic sensor  220  are provided. The shield member  300  includes the first opening window  331  provided between the first ultrasonic sensor  210  and the sheet  1  transported to the platen  11 . The shield member  300  also includes the second opening window  332  provided between the second ultrasonic sensor  220  and the sheet  1  transported to the platen  11 . 
     Therefore, the first ultrasonic sensor  210  and the second ultrasonic sensor  220  are surrounded by the conductive shield member, thereby protecting the first ultrasonic sensor  210  and the second ultrasonic sensor  220  from external electromagnetic waves. Therefore, it is possible to suppress the disadvantage that the noise signal due to the electromagnetic wave is superimposed on the received signals output from the first ultrasonic sensor  210  and the second ultrasonic sensor  220 . Further, by providing the first opening window  331  and the second opening window  332 , the shield member  300  does not hinder the transmission and reception of ultrasonic waves between the first ultrasonic sensor  210  and the sheet  1 , and the transmission and reception of ultrasonic waves between the second ultrasonic sensor  220  and the sheet  1 . 
     In the ultrasonic apparatus  200  of the present embodiment, the distance Z S1  between the first opening window  331  and the transmission and reception surface  211  of the first ultrasonic sensor  210  is shorter than the near-field limit distance Z N  of the first ultrasonic sensor  210 . The distance Z S2  between the second opening window  332  and the transmission and reception surface  221  of the second ultrasonic sensor  220  is shorter than the near-field limit distance Z N  of the second ultrasonic sensor  220 . That is, in the present embodiment, the first opening window  331  is provided within the near-field limit distance Z N  from the transmission and reception surface  211 , and the second opening window  332  is provided within the near-field limit distance Z N  from the transmission and reception surface  221 . 
     As a result, it is possible to apply an ultrasonic wave having a strong sound pressure in the vicinity of the near-field limit distance Z N  to the sheet  1  from the first ultrasonic sensor  210  and the second ultrasonic sensor  220 , and to improve the S/N ratio in ultrasonic wave transmission and reception processing. 
     In the ultrasonic apparatus  200  of the present embodiment, the circuit board  240  is provided inside the shield member  300 . Therefore, it is possible to suppress the influence of external electromagnetic waves on the circuit board  240 , and it is possible to suppress deterioration in accuracy of ultrasonic wave transmission and reception processing due to noise. 
     In the ultrasonic apparatus  200  of the embodiment, the shield member  300  includes the first side surface portion  321  that is parallel to the YZ plane including the Y direction in which the first ultrasonic sensor  210  and the second ultrasonic sensor  220  are arranged, and the Z direction in which ultrasonic waves are transmitted and received by the first ultrasonic sensor  210  and the second ultrasonic sensor  220 , and the second side surface portion  322  facing the first side surface portion  321 . The circuit board  240  is disposed such that the projected area of the circuit board  240  projected on the YZ plane is 70% or more of the area of the circuit board  240 . Accordingly, in the ultrasonic apparatus  200 , the ultrasonic apparatus  200  can be downsized as compared with the case where the circuit board  240  is disposed parallel to the XZ plane or the XY plane. 
     In the present embodiment, the circuit board  240  is disposed closer to the first side surface portion  321  than the midpoint between the first side surface portion  321  and the second side surface portion  322 . Further, between the first opening window  331  and the first ultrasonic sensor  210 , the first protective member  251  having the first protective surface  251 A provided with a plurality of first hole portions (voids  254 ) through which ultrasonic waves pass is provided. Further, between the second opening window  332  and the second ultrasonic sensor  220 , the second protective member  252  having the second protective surface  252 A provided with a plurality of second hole portions (voids  254 ) through which ultrasonic waves pass is provided. The first protective surface  251 A is inclined to a direction in which the ultrasonic wave transmitted from the first ultrasonic sensor  210  is reflected toward the second side surface portion  322  with respect to the ultrasonic wave transmission and reception surface  211  of the first ultrasonic sensor  210 . The second protective surface  252 A is inclined to a direction in which the ultrasonic wave transmitted from the second ultrasonic sensor  220  is reflected toward the second side surface portion  322  with respect to the ultrasonic wave transmission and reception surface  221  of the second ultrasonic sensor  220 . 
     As a result, with the first protective member  251  and the second protective member  252 , it is possible to suppress the entry of foreign matter such as ink droplets and paper dust into the inside of the shield member  300 , and it is possible to suppress adhesion of foreign matter to the first ultrasonic sensor  210  and the second ultrasonic sensor  220 . Therefore, it is possible to suppress the deterioration of the performance of the first ultrasonic sensor  210  or the second ultrasonic sensor  220 , that is, the decrease in the sound pressure of the transmitted ultrasonic wave and the decrease in the reception sensitivity of the ultrasonic wave. Further, since the first protective surface  251 A is inclined with respect to the transmission and reception surface  211  of the first ultrasonic sensor  210 , it is possible to suppress the disadvantage that ultrasonic waves are multiply reflected between the first ultrasonic sensor  210  and the first protective member  251 . Similarly, since the second protective surface  252 A is inclined with respect to the transmission and reception surface  221  of the second ultrasonic sensor  220 , it is possible to suppress the disadvantage that ultrasonic waves are multiply reflected between the second ultrasonic sensor  220  and the second protective member  252 . Accordingly, the ultrasonic apparatus  200  can suppress the disadvantage that a noise signal due to the occurrence of multiple reflection is superimposed on a received signal, and the first control circuit  241  and the second control circuit  242  can accurately calculate the first distance and the second distance. Further, part of the ultrasonic waves transmitted from the first ultrasonic sensor  210  and the second ultrasonic sensor  220  is reflected toward the second side surface portion  322  by the first protective surface  251 A and the second protective surface  252 A. As a result, it is possible to suppress the disadvantage that the ultrasonic waves that are noise components are input to the first ultrasonic sensor  210  and the second ultrasonic sensor  220 . That is, when the ultrasonic waves reflected by the first protective surface  251 A and the second protective surface  252 A travel toward the circuit board  240  that is close to the first ultrasonic sensor  210  and the second ultrasonic sensor  220 , most of the ultrasonic waves re-reflected by the circuit board  240  enter the first ultrasonic sensor  210  and the second ultrasonic sensor  220 . Since this ultrasonic wave is not the ultrasonic wave reflected by the sheet  1 , the ultrasonic wave becomes a noise component. On the other hand, in the present embodiment, part of the ultrasonic waves transmitted from the first ultrasonic sensor  210  and the second ultrasonic sensor  220  is reflected by the first protective surface  251 A and the second protective surface  252 A toward the second side surface portion  322  that is farther from the first ultrasonic sensor  210  and the second ultrasonic sensor  220  than the circuit board  240 . Therefore, compared to the case where the ultrasonic waves are reflected by the circuit board  240 , the amount of ultrasonic waves reflected by the second side surface portion  322  and input to the first ultrasonic sensor  210  and the second ultrasonic sensor  220  is reduced. Therefore, the ultrasonic apparatus  200  can suppress a decrease in the S/N ratio of the received signal, and can perform ultrasonic wave transmission and reception processing with high accuracy. 
     The ultrasonic apparatus  200  of the present embodiment includes the first holder  261  to which the first protective member  251  is attached and the second holder  262  to which the second protective member  252  is attached. The first holder  261  and the second holder  262  are detachably provided on the holder holding portion  304  of the shield member  300 . Therefore, when the first protective member  251  and the second protective member  252  are replaced, the first holder  261  and the second holder  262  can be easily removed from the shield member  300 , and the ultrasonic apparatus  200  can be easily maintained. 
     In the ultrasonic apparatus  200  of the present embodiment, the first holder  261  includes the first passage hole  261 A through which the ultrasonic waves that passed through the first protective member  251  pass, and the second holder  262  includes the second passage hole  262 A through which the ultrasonic waves that passed through the second protective member  252  pass. The opening size S S1  of the first opening window  331 , the opening size S m1  of the first passage hole  261 A, and the width S TR1  of the transmission and reception surface  211  of the first ultrasonic sensor  210  satisfy the relationship of S TR1 ≤S m1 ≤S S1 . In addition, the opening size S S2  of the second opening window  332 , the opening size S m2  of the second passage hole  262 A, and the width S TR2  of the transmission and reception surface  221  of the second ultrasonic sensor  220  satisfy the relationship of S TR2 ≤S m2 ≤S S2 . Thereby, the ultrasonic waves transmitted from the first ultrasonic sensor  210  and the second ultrasonic sensor  220  are not reflected by the first holder  261 , the second holder  262 , and the shield member  300 , and multiple reflection can be suppressed. Further, since it is possible to suppress the decrease in the sound pressure of the ultrasonic wave used for the measurement, it is possible to improve the S/N ratio of the received signal. 
     The printing apparatus  100  of the present embodiment includes the ultrasonic apparatus  200  and the controller  160 . Then, the controller  160  functions as the detector  164  that detects wrinkles, which is one of the abnormalities of the sheet  1 , based on the error signal output from the ultrasonic apparatus  200 . Accordingly, the printing apparatus  100  can detect the wrinkles occurring on the sheet  1  based on the error signal output from the ultrasonic apparatus  200 . 
     The printing apparatus  100  according to the present embodiment includes a transporter  120  that transports the sheet  1  along the Y direction which is the transport direction. The second ultrasonic sensor  220  provided in the ultrasonic apparatus  200  is disposed in the upstream in the Y direction with respect to the disposition position of the first ultrasonic sensor  210 . That is, the first ultrasonic sensor  210  and the second ultrasonic sensor  220  are disposed side by side along the Y direction. As a result, the printing apparatus  100  can suitably detect wrinkles that occur only in the upstream in the transport direction of the sheet  1  and wrinkles that occur only in the downstream. Accordingly, for example, wrinkles of the sheet  1  having various patterns as shown in  FIGS. 13 to 15  can be detected. 
     The printing apparatus  100  of the present embodiment includes the movement mechanism  150  for moving the ultrasonic apparatus  200  in the X direction intersecting with the Y direction. Accordingly, by moving the first ultrasonic sensor  210  and the second ultrasonic sensor  220  disposed side by side in the Y direction in the X direction, ultrasonic waves can be scanned along the X direction, and wrinkles can be detected over a wide area of the sheet  1 . 
     The printing apparatus  100  of the present embodiment includes a printer  141  that forms an image on the sheet  1 , and the first controller  163  and the second controller  165  control printing by the printer  141  based on the wrinkle detection result by the detector  164 . As a result, when the sheet  1  has wrinkles, printing can be interrupted, thereby suppressing ink consumption for printing. 
     Second Embodiment 
     In the above-described first embodiment, in the ultrasonic apparatus  200 , an example in which the first protective member  251  and the second protective member  252  are inclined so as to reflect ultrasonic waves toward the second side surface portion  322  on which the circuit board  240  is not disposed has been shown. On the other hand, in the ultrasonic apparatus  200  of the second embodiment, the inclination directions of the first protective member  251  and the second protective member  252  differ from those of the first embodiment. In the following description, the same reference numerals will be given to the configurations already described, and the description thereof will be omitted or simplified. 
       FIG. 16  is a cross-sectional view of an ultrasonic apparatus  200 A of the second embodiment taken along the YZ plane. As shown in  FIG. 16 , the ultrasonic apparatus  200 A of the present embodiment includes a first ultrasonic sensor  210 , a second ultrasonic sensor  220 , a first pedestal portion  231 , a second pedestal portion  232 , a circuit board  240 , a first protective member  251 , a second protective member  252 , a first holder  271 , a second holder  272 , and a shield member  300 . The first ultrasonic sensor  210  is fixed to the first pedestal portion  231  as in the first embodiment. Then, by fixing the first pedestal portion  231  to the shield member  300 , the first ultrasonic sensor  210  is provided inside the shield member  300  so as to face the first opening window  331 . The second ultrasonic sensor  220  is fixed to the second pedestal portion  232  as in the first embodiment. Then, by fixing the second pedestal portion  232  to the shield member  300 , the second ultrasonic sensor  220  is provided inside the shield member  300  so as to face the second opening window  332 . Similar to the first embodiment, the circuit board  240  is disposed parallel to the first side surface portion  321  and the second side surface portion  322 , and closer to the first side surface portion  321  than the midpoint between the first side surface portion  321  and the second side surface portion  322 . 
     The first protective member  251  is fixed to the first holding surface  273  of the first holder  271 . The first holding surface  273  of the first holder  271  is provided with a first passage hole  271 A penetrating in the Z direction. Then, the first holder  271  is engaged with the first engaging portion  304 A provided in the holder holding portion  304  of the shield member  300 , as in the first embodiment. Accordingly, the first protective member  251  is disposed between the first ultrasonic sensor  210  and the first opening window  331 . The second protective member  252  is fixed to the second holding surface  274  of the second holder  272 . The second holding surface  274  of the second holder  272  is provided with a second passage hole  272 A penetrating in the Z direction. Then, the second holder  272  is engaged with the second engaging portion  304 B provided in the holder holding portion  304  of the shield member  300 , as in the second embodiment. Accordingly, the second protective member  252  is disposed between the second ultrasonic sensor  220  and the second opening window  332 . 
     Here, in the present embodiment, the first protective member  251  held by the first holding surface  273  of the first holder  271  is inclined so that the distance from the transmission and reception surface  211  of the first ultrasonic sensor  210  increases toward the +Y side. That is, when the distance between any fifth point of the first protective surface  251 A and the transmission and reception surface  211  is L5, and the distance between a sixth point on the +Y side farther from the second ultrasonic sensor  220  than the fifth point and the transmission and reception surface  211  is L6, L5&lt;L6 is satisfied. In other words, the first protective member  251  is inclined so that the ultrasonic wave transmitted from the first ultrasonic sensor  210  in the Z direction is reflected by the first protective surface  251 A toward the third side surface portion  323  opposite to the second ultrasonic sensor  220 . 
     The second protective member  252  held by the second holding surface  274  of the second holder  272  is inclined so that the distance between the second protective surface  252 A and the transmission and reception surface  221  increases toward the −Y side. That is, when the distance between any seventh point of the second protective surface  252 A and the transmission and reception surface  221  is L7, and the distance between an eighth point on the Y side farther from the first ultrasonic sensor  210  than the seventh point and the transmission and reception surface  221  is L8, L7&lt;L8 is satisfied. In other words, the second protective member  252  is inclined so that the ultrasonic waves transmitted from the second ultrasonic sensor  220  in the Z direction are reflected by the second protective surface  252 A toward the fourth side surface portion  324  opposite to the first ultrasonic sensor  210 . 
     Thus, in the present embodiment, it is possible to suppress the disadvantage that the ultrasonic waves transmitted from the first ultrasonic sensor  210  are input to the second ultrasonic sensor  220 , and the ultrasonic waves transmitted from the second ultrasonic sensor  220  are input to the first ultrasonic sensor  210 . 
     Operational Effects of Present Embodiment 
     The present embodiment has the same operational effects as the first embodiment, and further has the following operational effects. In the ultrasonic apparatus  200  of the present embodiment, the first protective surface  251 A of the first protective member  251  is inclined with respect to the transmission and reception surface  211  of the first ultrasonic sensor  210  so as to reflect the ultrasonic waves transmitted from the first ultrasonic sensor in a direction away from the second ultrasonic sensor  220 . Further, the second protective surface  252 A of the second protective member  252  is inclined with respect to the transmission and reception surface  221  of the second ultrasonic sensor  220  so as to reflect the ultrasonic waves transmitted from the second ultrasonic sensor  220  in a direction away from the first ultrasonic sensor  210 . For this reason, even when the ultrasonic wave transmitted from the first ultrasonic sensor  210  is reflected by the first protective member  251 , the input of the reflected ultrasonic wave to the second ultrasonic sensor  220  can be suppressed. Further, even when the ultrasonic wave transmitted from the second ultrasonic sensor  220  is reflected by the second protective member  252 , the input of the reflected ultrasonic wave to the first ultrasonic sensor  210  can be suppressed. This suppresses the disadvantage that the received signal contains noise, and the circuit board  240  can accurately calculate the first distance and the second distance. 
     MODIFICATION EXAMPLE 
     The present disclosure is not limited to the above-described embodiments, and modifications, improvements, or the like within the scope of achieving the object of the present disclosure are included in the present disclosure. 
     Modification Example 1 
     In the first embodiment and the second embodiment, an example in which two ultrasonic sensors, the first ultrasonic sensor  210  and the second ultrasonic sensor  220 , are provided along the Y direction has been shown, but the present disclosure is not limited thereto. For example, three or more ultrasonic sensors may be provided along the Y direction. In this case, the circuit board  240  is provided with a sensor control circuit for each ultrasonic sensor. Each sensor control circuit causes the corresponding ultrasonic sensor to perform ultrasonic wave transmission and reception processing, and calculates the distance from the ultrasonic sensor to the sheet  1  based on the transmission and reception result. In addition, the determination circuit  243  outputs an error signal when any one of the distances between the ultrasonic sensors and the sheet  1  has a difference having the threshold value or more with respect to the distance measured by another ultrasonic sensor. 
     A plurality of ultrasonic sensors may be provided along the X direction, or a plurality of ultrasonic sensors may be provided in each of the Y direction and the X direction. In the case where a large number of ultrasonic sensors are disposed along each of the X direction and the Y direction, a movement mechanism for moving the ultrasonic apparatus may not be required. 
     Modification Example 2 
     In the first and second embodiments described above, the rectangular parallelepiped shield member  300  having the top surface portion  310 , the side surface portions  321  to  324 , and the bottom surface portion  330  is illustrated, but the configuration of the shield member is not limited thereto. For example, the shield member may be formed in another shape such as a cylindrical shape. 
     Modification Example 3 
     In the first embodiment and the second embodiment described above, a configuration in which the first ultrasonic sensor  210 , the second ultrasonic sensor  220 , and the circuit board  240  are provided inside one shield member  300  has been illustrated, but for example, the configuration may include a first shield member in which the first ultrasonic sensor  210  is provided, a second shield member in which the second ultrasonic sensor  220  is provided, and a third shield member in which the circuit board  240  is provided. 
     Modification Example 4 
     Although the ultrasonic apparatus  200  is fixed to the carriage  140  in the first and second embodiments, the ultrasonic apparatus  200  may be provided separately from the carriage  140 . In this case, it is preferable to separately provide a second movement mechanism for moving the ultrasonic apparatus  200  in the X direction. 
     Modification Example 5 
     In the above-described first and second embodiments, an example in which the printing apparatus  100  also functions as a detection apparatus including the ultrasonic apparatus  200  has been shown, but the present disclosure is not limited thereto. For example, an image scanner or the like that captures an image printed on a sheet that is an object as image data may be configured to include the detection apparatus. In such an image scanner, in order to read the image on the sheet, the sheet is transported to the image reading position, and image reading processing is performed by the scanner at the image reading position. At this time, when the sheet has wrinkles at the image reading position, the image cannot be properly captured, and the wrinkles are reflected in the captured image. Therefore, the ultrasonic apparatus according to the present disclosure may be incorporated in such an image scanner to detect wrinkles on the sheet at the image reading position. 
     Modification Example 6 
     In the above-described first and second embodiments, the detection apparatus that detects the wrinkles of an object as an abnormality by using the ultrasonic apparatus  200  is illustrated, but the disclosure is not limited thereto. For example, the ultrasonic apparatus  200  may be incorporated in a distance measuring apparatus that measures the distance to the object. In this case, when an error signal is output from the circuit board of the ultrasonic apparatus  200 , the distance measuring apparatus determines that the distance measuring accuracy is low and stops the measurement. When an error signal is not output, the distance measuring apparatus outputs the average of the first distance and the second distance as the distance between the ultrasonic apparatus and the object. 
     Modification Example 7 
     In the first and second embodiments, an example in which the width S TR1  of the transmission and reception surface  211  of the first ultrasonic sensor  210 , the opening size S S1  of the first opening window  331 , and the opening size S m1  of the first passage hole  261 A satisfy the relationship of S TR1  S m1 ≤S S1  has been shown, but the present disclosure is not limited thereto. For example, a plurality of ultrasonic transducers Tr constituting the first ultrasonic sensor  210  may be driven independently of each other, and the circuit board  240  may control the drive timing of each ultrasonic transducer Tr to form an ultrasonic beam that converges at a predetermined focus position. In this case, by controlling the first ultrasonic sensor  210  so that the platen  11  is at the focus position, the beam diameter of the ultrasonic waves becomes smaller toward the platen  11 . Therefore, the first opening window  331  and the first passage hole  261 A may be provided so as to satisfy S TR1 ≥S m1 ≥S S1 . The relationship between the width S TR2  of the transmission and reception surface  221  of the second ultrasonic sensor  220 , the opening size S S2  of the second opening window  332 , and the opening size S m2  of the second passage hole  262 A is the same. 
     Modification Example 8 
     In the first and second embodiments described above, an example in which the relationship between the distance Z S1  between the transmission and reception surface  211  of the first ultrasonic sensor  210  and the first opening window  331  and the near-field limit distance Z N  of the ultrasonic waves transmitted from the first ultrasonic sensor  210  satisfies Z S1 &lt;Z N  has been shown. 
     On the other hand, Z S1 =Z N  may be set. In the case of long-distance sound waves, the sound pressure distribution in the beam cross section of an ultrasonic beam becomes a distribution in which the sound pressure becomes weaker toward the periphery around the transmission center axis of the ultrasonic wave, that is, a simple sound pressure distribution. Therefore, when the sound pressure of the ultrasonic waves transmitted from the first ultrasonic sensor  210  is sufficiently large and the sound pressure in the far field can be maintained above a predetermined value, the distance Z S1  between the first opening window  331  and the transmission and reception surface  211  of the first ultrasonic sensor  210  may have the same dimension as the near-field limit distance Z N , or may be longer than the near-field limit distance Z N . The same applies to the distance between the second ultrasonic sensor  220  and the second opening window  332 . 
     Modification Example 9 
     In the first embodiment and the second embodiment, the configuration in which the first protective member  251  is held by the first holder  261  and the second protective member  252  is held by the second holder  262  has been exemplified, but the first protective member  251  and the second protective member  252  may be directly fixed to the vicinity of the first opening window  331  and the second opening window  332  of the shield member  300 . 
     Also, an example is shown in which the first holders  261  and  271 , and the second holders  262  and  272  are detachably attached to the holder holding portion  304  of the shield member  300 , but the first holders  261  and  271 , and the second holders  262  and  272  may be fixed to the holder holding portion  304 . 
     Modification Example 10 
     Although the circuit board  240  is provided parallel to the first side surface portion  321  and the second side surface portion  322  and in the vicinity of the first side surface portion  321 , the present disclosure is not limited thereto. For example, the circuit board  240  may be provided at a midpoint between the first side surface portion  321  and the second side surface portion  322 . Further, as described in Modification Example 1, when the plurality of ultrasonic sensors are disposed along the X direction, the circuit board  240  is disposed parallel to the third side surface portion  323  and the fourth side surface portion  324 . Further, when the plurality of ultrasonic sensors are disposed in an array along the X direction and the Y direction, the circuit board  240  may be disposed in the vicinity of the top surface portion  310  and parallel to the top surface portion  310 . 
     Overview of the Disclosure 
     According to the first aspect, there is provided an ultrasonic apparatus including a first ultrasonic sensor that transmits an ultrasonic wave to an object and receives the ultrasonic wave reflected by the object, a second ultrasonic sensor that transmits an ultrasonic wave to the object and receives the ultrasonic wave reflected by the object, an error output portion that outputs an error signal when the difference between a first distance between the first ultrasonic sensor and the object calculated based on ultrasonic wave transmission and reception processing using the first ultrasonic sensor and a second distance between the second ultrasonic sensor and the object calculated based on ultrasonic wave transmission and reception processing using the second ultrasonic sensor is equal to or greater than a threshold value. 
     As a result, the ultrasonic apparatus can detect an abnormality in an object and output an error signal by performing ultrasonic wave transmission and reception processing at a plurality of positions of the object. In this case, by transmitting the ultrasonic waves to one location of the object, it is possible to determine an abnormality more accurately than the ultrasonic apparatus of related art that detects an abnormality of the object. 
     The ultrasonic apparatus of the first aspect further includes a conductive shield member in which the first ultrasonic sensor and the second ultrasonic sensor are provided, and it is preferable that the shield member includes a first opening window and a second opening window, the first opening window is provided between the first ultrasonic sensor and the object, and the second opening window is provided between the second ultrasonic sensor and the object. 
     As a result, the ultrasonic apparatus can apply an ultrasonic wave having a strong sound pressure in the vicinity of the near-field limit distance to the object from the first ultrasonic sensor and the second ultrasonic sensor, and can improve the S/N ratio in the ultrasonic wave transmission and reception processing. 
     In the ultrasonic apparatus of this aspect, it is preferable that a distance between the first opening window and a transmission and reception surface of the first ultrasonic sensor is shorter than a near-field limit distance of the first ultrasonic sensor, and a distance between the second opening window and a transmission and reception surface of the second ultrasonic sensor is shorter than a near-field limit distance of the second ultrasonic sensor. 
     As a result, the ultrasonic apparatus can apply an ultrasonic wave having a strong sound pressure in the vicinity of the near-field limit distance to the object from the first ultrasonic sensor and the second ultrasonic sensor, and can improve the S/N ratio in the ultrasonic wave transmission and reception processing. 
     The ultrasonic apparatus according to the present aspect includes the circuit board that is provided with the error output portion, and it is preferable that the circuit board is provided inside the shield member. Therefore, the ultrasonic apparatus can suppress the influence of external electromagnetic waves on the circuit board, and can suppress deterioration in the accuracy of ultrasonic wave transmission and reception processing due to noise. 
     In the ultrasonic apparatus according to the present aspect, it is preferable that the shield member includes a first side surface portion parallel to a plane including a first direction in which the first ultrasonic sensor and the second ultrasonic sensor are arranged, and a transmission and reception direction of ultrasonic waves in which ultrasonic waves are transmitted from the first ultrasonic sensor and the second ultrasonic sensor, and a second side surface portion facing the first side surface portion, the circuit board is disposed closer to the first side surface portion than a midpoint between the first side surface portion and the second side surface portion, a first protective member having a first protective surface provided with a plurality of first hole portions for passing ultrasonic waves is provided between the first opening window and the first ultrasonic sensor, a second protective member having a second protective surface provided with a plurality of second hole portions for passing ultrasonic waves is provided between the second opening window and the second ultrasonic sensor, the first protective surface is inclined with respect to the transmission and reception surface of the first ultrasonic sensor in a direction in which ultrasonic waves transmitted from the first ultrasonic sensor are reflected toward the second side surface portion, and the second protective surface is inclined with respect to the transmission and reception surface of the second ultrasonic sensor in a direction in which ultrasonic waves transmitted from the second ultrasonic sensor are reflected toward the second side surface portion. 
     As a result, the first ultrasonic sensor and the second ultrasonic sensor can be protected by the first protective member and the second protective member. That is, it is possible to suppress the entry of foreign matter into the inside of the shield member through the first opening window and the second opening window, and to prevent the foreign matter from adhering to the first ultrasonic sensor and the second ultrasonic sensor. In addition, when such a first protective member or a second protective member is provided, part of the ultrasonic waves transmitted from the first ultrasonic sensor and the second ultrasonic sensor is reflected by the first protective surface and the second protective surface. Here, in the present aspect, the circuit board is disposed closer to the first side surface portion than the midpoint between the first side surface portion and the second side surface portion. Then, part of the ultrasonic waves reflected by the first protective surface and the second protective surface is reflected toward the second side surface portion opposite to the first side surface portion where the circuit board is disposed in close proximity. With such a configuration, it is possible to suppress the disadvantage that part of the ultrasonic waves reflected by the first protective surface and the second protective surface is multiply reflected in the shield and returns to the ultrasonic sensor. That is, when part of the ultrasonic waves reflected by the first protective surface or the second protective surface is directed to the first side surface portion, the ultrasonic waves are reflected on the circuit board disposed closer to the first ultrasonic sensor and the second ultrasonic sensor than the first side surface portion. In this case, the amount of ultrasonic components that are re-reflected by the circuit board and enter the ultrasonic sensor increases. On the other hand, the second side surface portion is farther from the first ultrasonic sensor and the second ultrasonic sensor than the circuit board. Therefore, even if the ultrasonic waves are re-reflected on the second side surface portion, compared to the case where the ultrasonic waves are re-reflected on the circuit board, the ultrasonic component entering the first ultrasonic sensor or the second ultrasonic sensor can be reduced. Therefore, it is possible to suppress an increase in noise due to an unnecessary reflected ultrasonic wave component, and it is possible to suppress a decrease in the S/N ratio of the received signal. 
     In the ultrasonic apparatus according to the present aspect, it is preferable that the shield member includes a first side surface portion parallel to a plane including a first direction in which the first ultrasonic sensor and the second ultrasonic sensor are arranged, and a transmission and reception direction of ultrasonic waves in which ultrasonic waves are transmitted from the first ultrasonic sensor and the second ultrasonic sensor, and a second side surface portion facing the first side surface portion, and the projected area of the circuit board onto the plane is 70% or more of the area of the circuit board. That is, it is preferable that the angle formed by the board surface of the circuit board and the plane parallel to the first side surface portion and the second side surface portion is 0° or more and 45° or less. This makes it possible to reduce the size of the ultrasonic apparatus as compared with the case where the circuit board is disposed orthogonal to the direction in which the first ultrasonic sensor and the second ultrasonic sensor are disposed. 
     In the ultrasonic apparatus of the present aspect, it is preferable that a first protective member having a plurality of first hole portions for passing ultrasonic waves is provided between the first opening window and the first ultrasonic sensor, and a second protective member having a plurality of second hole portions for passing ultrasonic waves is provided between the second opening window and the second ultrasonic sensor. 
     As a result, the first protective member and the second protective member can suppress the entry of foreign matter such as ink droplets and paper dust into the inside of the shield member, and can suppress adhesion of foreign matter to the first ultrasonic sensor and the second ultrasonic sensor. Therefore, it is possible to suppress the deterioration of the performance of the first ultrasonic sensor and the second ultrasonic sensor, that is, the decrease in the sound pressure of the transmitted ultrasonic wave and the decrease in the reception sensitivity of the ultrasonic wave. 
     In the ultrasonic apparatus of the present aspect, it is preferable that the first protective member includes a first protective surface provided with the plurality of first hole portions, the first protective surface is inclined with respect to the transmission and reception surface of the first ultrasonic sensor, the second protective member includes a second protective surface provided with the plurality of second hole portions, and the second protective surface is inclined with respect to the transmission and reception surface of the second ultrasonic sensor. 
     As a result, the ultrasonic apparatus can suppress the disadvantage that ultrasonic waves are multiply reflected between the first ultrasonic sensor and the first protective member, and can suppress the disadvantage that ultrasonic waves are multiply reflected between the second ultrasonic sensor  220  and the second protective member  252 . Therefore, the ultrasonic apparatus can suppress the disadvantage that the noise signal due to the occurrence of multiple reflection is superimposed on the received signal, and can accurately calculate the first distance and the second distance based on the ultrasonic wave transmission and reception processing by the first ultrasonic sensor and the second ultrasonic sensor. 
     In the ultrasonic apparatus of the present aspect, it is preferable that the first protective surface is inclined with respect to the transmission and reception surface of the first ultrasonic sensor so as to reflect the ultrasonic waves transmitted from the first ultrasonic sensor in a direction away from the second ultrasonic sensor, and the second protective surface is inclined with respect to the transmission and reception surface of the second ultrasonic sensor so as to reflect the ultrasonic waves transmitted from the second ultrasonic sensor in a direction away from the first ultrasonic sensor. 
     As a result, even when the ultrasonic wave transmitted from the first ultrasonic sensor is reflected by the first protective member, it is possible to suppress the disadvantage that the reflected ultrasonic wave is input to the second ultrasonic sensor. Further, even when the ultrasonic wave transmitted from the second ultrasonic sensor is reflected by the second protective member, it is possible to suppress the disadvantage that the reflected ultrasonic wave is input to the first ultrasonic sensor. As a result, the disadvantage that the received signal contains noise can be suppressed, and the measurement accuracy of the ultrasonic apparatus can be improved. 
     The ultrasonic apparatus according to the present aspect includes a first holder to which the first protective member is attached, and a second holder to which the second protective member is attached, and it is preferable that the first holder and the second holder are detachably attached to the shield member. 
     Thus, when the first protective member and the second protective member are replaced, the first holder and the second holder can be easily removed from the shield member, and the ultrasonic apparatus can be easily maintained. 
     In the ultrasonic apparatus  200  according to the present aspect, it is preferable that the first holder includes a first passage hole through which ultrasonic waves that passed through the first protective member pass, the second holder includes a second passage hole through which ultrasonic waves that passed through the second protective member pass, an opening size S S1  of the first opening window, an opening size S m1  of the first passage hole, and a width S TR1  of the transmission and reception surface of the first ultrasonic sensor satisfy S TR1  S m1 ≤S S1 , and an opening size S S2  of the second opening window, an opening size S m2  of the second passage hole, and a width S TR2  of the transmission and reception surface of the second ultrasonic sensor satisfy S TR2 ≤S m2 ≤S S2 . 
     As a result, the ultrasonic waves transmitted from the first ultrasonic sensor and the second ultrasonic sensor are not reflected by the first holder, the second holder and the shield member, and multiple reflection can be suppressed. Further, it is possible to suppress a decrease in the sound pressure of ultrasonic waves input to the object, and it is possible to improve the S/N ratio of the received signal. 
     The detection apparatus of the second aspect includes the ultrasonic apparatus of the first aspect, and a detector that detects an abnormality of the object based on the error signal output from the ultrasonic apparatus. Accordingly, the detection apparatus can detect the abnormality of the object based on the error signal output from the ultrasonic apparatus. 
     The detection apparatus according to the present aspect includes a transport mechanism for transporting the object along a predetermined transport direction, and it is preferable that the second ultrasonic sensor is disposed on an upstream of a disposition position of the first ultrasonic sensor in the transport direction. Accordingly, the detection apparatus can suitably detect an abnormality that occurs when an object is transported and that occurs only in the upstream in the transport direction, and an abnormality that occurs only in the downstream. 
     In the detection apparatus according to the present aspect, it is preferable to further include a movement mechanism for moving the ultrasonic apparatus in a direction intersecting with the transport direction. Thereby, by moving the first ultrasonic sensor and the second ultrasonic sensor disposed side by side in the transport direction in a direction intersecting the transport direction, it is possible to detect an abnormality of the object in a wide range. 
     According to a third aspect, there is provided a printing apparatus that includes a detection apparatus according to the second aspect and a printer that forms an image on the object, and controls printing by the printer based on a detection result of the abnormality by the detector. As a result, when the object has an abnormality such as wrinkles, printing by the printer can be interrupted, thereby suppressing ink consumption for printing.