Abstract:
A double feed detection method for detecting a double feed of a sheet-like detection object transported through a processing unit has an ultrasonic transmitter and an ultrasonic receiver positioned so as to sandwich a transport path along which the sheet-like detection object is transported so as to detect a double feed of the detection object. The double feed detection method has a step of forming a gap between the sheet-like detection objects where the sheet-like detection objects overlap when a double feed of the sheet-like detection objects occurs. Forming the gap between the sheets enhances an attenuation of an intensity of a response signal whenever a double feed occurs. As a result, a distinction between the intensity of the response signal when a double feed has occurred and when no double feed has occurred is sharpened and thus more easily detected, improving the double feed detection accuracy.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates generally to a double feed detection method and device, and more particularly, to a double feed detection method and device in which an ultrasonic sensor is used to detect double feeding of a sheet-like detection object in the course of transport.  
           [0003]    2. Description of Related Art  
           [0004]    In the case of an image scanner or an optical character reader (OCR), the image scanner or OCR reads lettering and the like printed on both sides of the paper after the paper has been separated into single sheets. However, if by some mistake two sheets remain unseparated and are transported onward in that double-fed state, then the image scanner or OCR is unable to read the two overlapped surfaces. Moreover, if a certain number of sheets are to be read in order, then such double feeding disrupts the accuracy of the count, which is undesirable. As a result, it is important not only to prevent the occurrence of double-fed sheets but also to detect accurately such double feeds when they occur and to deal with them promptly.  
           [0005]    It should be noted that the detection of double feeds is not limited to sheets of paper, as described above, but may encompass any sheet-like object which should be fed through a transport unit one sheet at a time, such as plastic, metal, and so forth.  
           [0006]    As conventional methods for detecting a double feed, there exist those that use optical sensors and those that use ultrasonic sensors. However, in the case of optical sensors, the condition of the print on the printed surfaces of the paper can change the amount of light reflected from or passing through the paper, and so for this reason the ultrasonic sensor method is more commonly used because the detection values do not change depending on the condition of the print.  
           [0007]    [0007]FIG. 1 is a lateral cross-sectional view of a conventional double feed detection device utilizing an ultrasonic sensor, in this case Japanese Laid-Open Patent Application No. 6-49537.  
           [0008]    The double feed detection device  1   a  is provided with a mesh-like support stand  2  through which ultrasonic waves pass. Sheets of paper  3   a  are carried along a transport route (not shown in the diagram) on a top surface of the support stand  2 . Placed a predetermined distance apart and sandwiching the upper surface of the support stand  2  are ultrasonic transmitter element  4   a  and ultrasonic receiver element  4   b,  disposed opposite each other. The ultrasonic receiver element  4   b  is mounted on an internal holder  5   b  of a cooling jacket case  5   a,  the internal holder  5   b  having a skirt  5   c  on a bottom part thereof.  
           [0009]    An annular nozzle  6  is provided in a space between the skirt  5   c  and a jacket outer wall  5   d,  such as to allow a flow of air, that is, an air curtain  7 , to be generated so as to press the paper  3   a  onto the support stand  2  and at the same time to shield the interior of the structure from externally generated heat, thus serving to eliminate fluctuation in detection values due to changes either in temperature or in the flatness of the surface of the paper  3   a  due to shaking.  
           [0010]    Additionally, though not shown in the diagrams, a sheet multi-feed detection device according to Japanese Laid-Open Patent Application No. 6-49567, comprising a pair of sheet retainers located near and pinching the lateral sides of a portion of a sheet feed path through which ultrasonic waves pass as well as detecting means for detecting when a sheet is retained by one or the other of the retaining means, is configured to detect a multi-feed state based on an attenuation of the ultrasonic sound waves when it is detected that a sheet is retained by one or the other of the retaining means.  
           [0011]    As will be appreciated by those of skill in the art, if the sheet of paper flutters, then the detection signal also becomes unreliable. In the structure described above, the sheet is caught by one or the other of the sheet retainers and detection takes place without the sheet fluttering, so an erroneous reading can be prevented.  
           [0012]    In both of the above-described cases, the devices involved are designed to ensure accurate detection of a double feed by for example eliminating the erroneous readings that are caused by fluctuation of the paper.  
           [0013]    As another example of the conventional art there is the double feed detection device according to Japanese Laid-Open Patent Application No. 5-40030, shown in FIG. 2.  
           [0014]    The double feed detection device  1   b  has an ultrasonic sound transmitter  8   a  that transmits ultrasonic sounds of a predetermined intensity and an ultrasonic sound receiver  8   b  disposed opposite each other and sandwiching the transport path through which the sheet-like detection object  3   b  passes. The intensity of the ultrasonic sounds detected by the ultrasonic sound receiver  8   b  are compared to a predetermined reference value, and if the detected intensity is at or below the reference value then it is determined that the detection object  3   b  is double fed and a signal indicating such double feed is output by a double feed discrimination circuit  9 .  
           [0015]    The above-described detection principle is based on the fact that the intensity of the ultrasonic sound that passes through the detection objects  3   b  when two such sheet-like detection objects  3   b  pass between the ultrasonic sound transmitter  8   a  and the ultrasonic sound receiver  8   b  is virtually constant regardless of the material or the thickness of the detection object  3   b,  which means that the need to make an initial adjustment each time the material changes can be eliminated.  
           [0016]    However, the double feed detection device according to Japanese Laid-Open Patent Application No. 5-40030 differs from the first two conventional examples described above and has a disadvantage in that, when detecting a double feed of sheet-like detection objects of different materials and thicknesses, the detection sensitivity, that is, the limit at which detection can be conducted without adjusting the ultrasonic sensor, is reduced.  
         SUMMARY OF THE INVENTION  
         [0017]    Accordingly, it is a general object of the present invention to provide an improved and useful double feed detection method and device, in which the above-described disadvantage is eliminated and double feed detection sensitivity is improved.  
           [0018]    Another more specific object of the present invention is to form an air gap between double-fed sheets so as to greatly reduce the amount of ultrasound that passes through the two as compared to when there is only one sheet present or two sheets stuck together without any air gap in between, thereby heightening the contrast between one sheet fed properly and two sheets that are double fed.  
           [0019]    The above-described objects of the present invention are achieved by a double feed detection method for detecting a double feed of a sheet-like detection object being transported through a processing unit, an ultrasonic transmitter and an ultrasonic receiver being positioned so as to sandwich a transport path along which the sheet-like detection object is transported so as to detect a double feed of the detection object, the double feed detection method comprising a step of forming a gap between the sheet-like detection objects where the sheet-like detection objects overlap when a double feed of the sheet-like detection objects occurs.  
           [0020]    Additionally, the above-described objects of the present invention are also achieved by a double feed detection device for detecting a double feed of a sheet-like detection object being transported through a processing unit using an ultrasonic sensor to detect a double feed of the detection object, the double feed detection device comprising:  
           [0021]    an ultrasonic transmitter and an ultrasonic receiver positioned so as to sandwich a transport path along which the sheet-like detection object is transported; and  
           [0022]    a gap-forming mechanism that forms a gap between the sheet-like detection objects where the sheet-like detection objects overlap when a double feed of the sheet-like detection objects occurs.  
           [0023]    Additionally, the above-described objects of the present invention are also achieved by an optical form reader comprising:  
           [0024]    a scanner including a part that optically reads information stored on a form; and  
           [0025]    a double feed detection device for detecting a double feed of a plurality of forms being transported through a processing unit using an ultrasonic sensor to detect a double feed of the detection object,  
           [0026]    the double feed detection device comprising:  
           [0027]    an ultrasonic transmitter and an ultrasonic receiver positioned so as to sandwich a transport path along which the forms are transported; and  
           [0028]    a gap-forming mechanism that forms a gap between the forms where the forms overlap when a double feed of the forms occurs.  
           [0029]    Other objects, features and advantages of the present invention will become more apparent from the following detailed description when read in conjunction with the accompanying drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0030]    [0030]FIG. 1 is a lateral cross-sectional view of a conventional double feed detection device;  
         [0031]    [0031]FIG. 2 is a structural diagram of another conventional double feed detection device;  
         [0032]    [0032]FIG. 3 is a lateral view of an optical scanner equipped with a double feed detection device according to one embodiment of the present invention;  
         [0033]    [0033]FIG. 4 is a flow chart illustrating an operation of the optical scanner shown in FIG. 3;  
         [0034]    [0034]FIG. 5 is a side view of a double feed detection device according to a first embodiment of the present invention;  
         [0035]    [0035]FIG. 6 is a circuit structure diagram of the double feed detection device shown in FIG. 5;  
         [0036]    [0036]FIGS. 7A and 7B are diagrams illustrating the double feed detection device shown in FIG. 5, in which FIG. 7A is a plan view of the double feed detection device shown in FIG. 5 and FIG. 7B shows a gear arrangement of the double feed detection device shown in FIG. 5;  
         [0037]    [0037]FIGS. 8A, 8B,  8 C and  8 D are diagrams showing sample changes with time in transmitted sound volume, output voltage inversion value, inverted output voltage peak hold value and comparator output, respectively;  
         [0038]    [0038]FIG. 9 is a lateral view of a double feed detection device according to a second embodiment of the present invention; and  
         [0039]    [0039]FIG. 10 is a plan view of a double feed detection device according to a third embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0040]    A description will now be given of embodiments of the present invention, with reference to the accompanying drawings. It should be noted that identical or corresponding elements in the embodiments are given identical or corresponding reference numbers in all drawings, with detailed descriptions of such elements given once and thereafter omitted.  
         [0041]    Additionally, though the following description refers to a double feed and its detection, it should be understood that the term “double feed” is meant to refer to any overlapping of two or more sheet-like detection objects, and as such is not limited to situations involving an overlap (either partial or complete) between two sheets but includes also overlaps (partial or complete) of three or more sheets.  
         [0042]    The term ultrasonic applies to sound waves above the range of human hearing, that is, frequencies in the range of 20-220 kHz. It is known that a portion of such sound waves are reflected at a boundary layer between media of different acoustic impedances. Acoustic impedance is a product of the speed of sound waves through the medium and the density of the medium, and therefore the difference in acoustic impedance between a solid such as paper on the one hand and air on the other, whose sound speeds and densities differ so sharply, is very great. By taking advantage of this difference and forming an air gap between doubled sheets of paper, approximately 99% of the ultrasonic waves that pass through the first sheet and the subsequent air gap are then reflected back from the surface of the second sheet, meaning that the amount of ultrasound that passes through the two sheets is greatly reduced as compared to when there is only one sheet present or two sheets stuck together without any air gap in between, thereby heightening the contrast between one sheet fed normally and two sheets that are double fed.  
         [0043]    By contrast, as described above, the volume of ultrasound that passes through one sheet and the volume of ultrasound that passes through two sheet that are stuck together is virtually identical.  
         [0044]    Moreover, in the case of frequencies in the audible range, the maximum above-described effect cannot be obtained.  
         [0045]    [0045]FIG. 3 is a lateral view of an optical scanner equipped with a double feed detection device according to one embodiment of the present invention.  
         [0046]    As shown in FIG. 3, a double feed detection device  10  is installed in a optical form reader  12 , the optical form reader  12  comprising a hopper  14 , a pick-up roller  16 , a form separator  18 , the double feed detection device  10 , a transport roller  20 , a transport path  22 , a stacker  24 , a form front-surface optical reading unit  26  and a form back-surface optical reading unit  28 .  
         [0047]    A description will now be given of an operation of the optical form reader  12 , with reference to FIG. 3 and FIG. 4.  
         [0048]    [0048]FIG. 4 is a flow chart illustrating an operation of the optical scanner shown in FIG. 3.  
         [0049]    A multiplicity of forms constituting the sheet-like detection objects  30  is set in the hopper  14 . The pick-up roller  16  takes the forms  30  one sheet at a time starting from the top of the stack and feeds the forms  30  onward in an X 1  direction to the form separator  18  in a step S 1 .  
         [0050]    If for some reason the forms  30  are double fed, then they are separated by the form separator  18  in a step S 2 . The forms  30  are then sent onward from the form separator  18  one at a time.  
         [0051]    However, it may happen that two forms are stuck together, for example by static electricity, and in that unseparated state emitted from the form separator  18 . In this case, the double feed detection device  10  detects the presence of double-fed forms  30  in a double feed check step S 3 , and flashes an alarm indicator and causes the optical form reader to stop in an stop or alarm step S 4 .  
         [0052]    On the other hand, a properly separated single form  30  passes through the double feed detection device  10 , is transported by the transport roller  20 , and the lettering or other information written on the front and back of the form  30  is read by the form front-surface optical reading unit  26  and the form back-surface optical reading unit  28  in a step S 5 , and the form  30  thereafter transported along the transport path  22  to the stacker  24 .  
         [0053]    The next form  30  is then taken from the hopper  14  and undergoes the same processing as described above.  
         [0054]    Descriptions will now be given in order of three different embodiments of the double-feed detection device  10  according to the present invention, with reference to the accompanying drawings.  
         [0055]    [0055]FIG. 5 is a side view of a double feed detection device according to a first embodiment of the present invention.  
         [0056]    As shown in FIG. 5, a double-feed detection device  10   a  according to the first embodiment of the present invention comprises an ultrasonic sensor  32  transmitter  32   a  and an ultrasonic sensor  32  receiver  32   b  disposed opposite each other so as to sandwich the transport path  22  on the X 1  side of the form separator  18 . Additionally, paired rollers  34   a,    34   b  and  36   a,    36   b  are provided at upstream and downstream sides, respectively, of the ultrasonic sensor  32 .  
         [0057]    The ultrasonic sensor  32  transmitter  32   a  is a transmitter, and the receiver  32   b  is a microphone. Ultrasonic sound waves of a predetermined intensity are generated from the transmitter  32   a,  attenuate after passing through the form  30  interposed in a path of propagation of the ultrasonic sound waves, and in that attenuated state are collected by the receiver  32   b.    
         [0058]    [0058]FIG. 6 is a circuit structure diagram of the double feed detection device shown in FIG. 5.  
         [0059]    The reception signal output from the receiver  32   b  is amplified by an amplifier  38 , after which it is compared to a cutoff signal (a reference signal) by a comparator  40 , and that output then sent to a microprocessor unit (MPU)  42 . If attenuation meets or exceeds a predetermined value, then an alarm is indicated at an operating panel  43  by a signal sent from the MPU  42 . At the same time, a stop signal is generated by a control unit  44  by a signal sent from the MPU  42 . The generation of the stop signal causes the optical form reader  12  to stop.  
         [0060]    [0060]FIGS. 7A and 7B are diagrams illustrating the double feed detection device shown in FIG. 5, in which FIG. 7A is a plan view of the double feed detection device shown in FIG. 5 and FIG. 7B shows a gear arrangement of the double feed detection device shown in FIG. 5.  
         [0061]    Rollers  34   a,    34   b  and rollers  36   a,    36   b  are driven by a single motor in order to make the device more compact, with the motor and gears laid out in the configuration shown for example in FIGS. 7A and 7B. The motor  46  is directly linked to a gear  48   a,  and the gear  48   a  is linked in order to gears  48   b,    48   c  and  48   d  in one direction and, in the other direction, is linked in order to gears  48   e,    48   f,    48   g    48   h,    48   i,  and  48   j.  Gear  48   h  and roller  34   a,  and gear  48   b  and roller  34   b,  and gear  48   j  and roller  36   a,  and gear  48   d  and roller  36   b  are each provided on the same shaft. In this case, the gears  48   h,    48   b  and  48   d  have the same diameter, with the gear  48   j  having a diameter that is slightly larger than that of gears  48   h,    48   b  and  48   d.    
         [0062]    The rollers  34   a,    34   b  and the rollers  36   a,    36   b  are in this case all of the same diameter. Therefore, when the gears are rotated by the motor  46 , only the roller  36   a,  which is directly linked to the gear  48   j  (the gear  48   j  having the slowest rotation speed), rotates more slowly than the other rollers  34   a,    34   b  and  36   b.    
         [0063]    A description will now be given of the double feed detection device  10   a  having the above-described structure according to the first embodiment of the present invention.  
         [0064]    As described previously, the multiplicity of forms  30  in the hopper  14  are transported one at a time in the X 1  direction to the separator  18  by the pick-up roller  16 .  
         [0065]    It occasionally happens that two forms  30  are stuck together and sent onward in that unseparated state. In such a case, the double-fed forms are separated by the form separator  18 . Then, the separated forms  30  are sent onward one at a time from the form separator  18 .  
         [0066]    However, if, for example, a plurality of forms  30  are stuck together by static electricity and fail to be separated by the form separator  18 , then these unseparated forms may be mistakenly sent onward from the form separator  18  to the double feed detection device  10   a.    
         [0067]    In this case, the coefficient of friction between the form  30  and the rollers  36   a  and  36   b  is set to be larger than the coefficient of friction between the double-fed forms  30 . As a result, a form  30   b  that contacts the roller  36   b  (the bottom form shown in FIG. 4) starts to slip with respect to the form  30   a  on top of the bottom form  30   b,  such that the rotation of the roller  36   b  transports the bottom form  30   b  at an ordinary transport speed in the X 1  direction onward to a next processing stage. At the same time, the top from  30   a,  which contacts the roller  36   a,  is sent onward by the roller  36   a  in the X 1  direction to the next processing stage at a transport speed slightly less than that of the roller  36   b.    
         [0068]    Accordingly, the top form  30   a,  which is sent onward at a transport speed that is slower than that of the bottom form  30   b,  gradually forms a bulge A upstream of the roller  36   a,  that is, on an X 2  side. If, for example, the ratio of the rotation speed of the roller  36   a  to the roller  36   b  is 30:29, then when the lower form  30   b  has advanced 30 mm the upper form  30   a  has formed a bulge A of 1 mm. Accordingly, at the portion of the bulge A, a slight gap C is formed between the upper and lower forms  30   a  and  30   b.  Thus the rollers  34   a,    34   b,    36   a  and  36   b  function as a gap-forming mechanism, forming a gap between double-fed forms  30   a  and  30   b.  In other words, the roller  34   a  acts as a brake and the roller  34   b  acts as a transport part. It should be noted that, depending on the dimensions of the form, when the distance between the form separator  18  and the rollers  32   a  and  34   b  is short, the rollers  32   a  and  34   b  can be eliminated so that just the rollers  36   a  and  36   b  form the gap-forming mechanism.  
         [0069]    Thus, a double feed of the forms  30  is detected by the ultrasonic sensor  32  using the acoustic pattern shown in FIGS. 8A, 8B and  8 C.  
         [0070]    [0070]FIGS. 8A, 8B,  8 C and  8 D are diagrams showing sample changes with time in transmitted sound volume, output voltage inversion value, inverted output voltage peak hold value and comparator output, respectively.  
         [0071]    [0071]FIG. 8A shows the amount of sound passing through the detection object (transmitted sound volume, measured in decibels) on the vertical axis and time (measured in seconds) on the horizontal axis.  
         [0072]    If for example a single N- 1  form 30 passes the ultrasonic sensor  32  detection point, then during a time period t 1 -t 2  (the time it takes for the form  30  to pass through the detection point), the transmitted sound volume, that is, the amount of sound passing through the form  30 , shows an attenuation of W 1 . (For convenience of description, the un-attenuated volume of sound transmitted when no form  30  passes through the detection point at a time prior to t 1  is taken as the reference acoustic level).  
         [0073]    During the time t 2  to t 3 , that is, during the time required for a succeeding N form  30  to reach the detection point at time t 3 , the transmitted sound volume hardly attenuates at all.  
         [0074]    When the N form  30  passes through the ultrasonic sensor  32  detection point during time t 3 -t 4 , the amount of sound passing through the N form  30  shows an attenuation of W 2  as shown in FIG. 8A. In this case, slight variations in the thicknesses of the N- 1  and N forms  30  as well as in their surface conditions lead to a slight difference in the amount of sound passing through the forms during detection, a difference indicated as Δ in FIG. 8A.  
         [0075]    When, for some reason, at a time t 5 , forms N+1 and N+2 reach the ultrasonic sensor  32  detection point in a double-fed state, the passage of these two overlapping forms  30  through the detection point at the same time results in an attenuation of the volume of sound passing through the forms.  
         [0076]    At this time, the upper and lower forms  30  are stuck together completely, with no gap therebetween, and in such state pass through the ultrasonic sensor  32  detection point at time intervals shown as t 5 -t 6  in FIG. 8A, causing an attenuation in transmitted sound volume shown as W 3  in FIG. 8A.  
         [0077]    As can be appreciated from FIG. 8A, the attenuation W 3  obtained at time interval t 5 -t 6 , obtained when the double-fed forms  30  pass the ultrasonic sensor  32  detection point with no gap formed between the forms, does not differ greatly from the attenuations W 1 , W 2 , obtained when N- 1  and N forms  30  pass the detection point as single sheets.  
         [0078]    By contrast, when the rollers  36   a,    36   b  that function as gap-forming means form a gap C between the upper and lower double-fed sheets  30   a,    30   b,  and when that portion of the double-fed forms  30   a,    30   b  in which the gap C is formed passes the detection point at time intervals t 6 -t 7  and again t 8 -t 9 , the sound transmitted from the transmitter  32   a  and shown as level S 1  in FIG. 5 passes through the lower sheet  30   b  to become a level S 2  as shown in FIG. 5, further passing through the air gap formed between the upper and lower forms  30   a,    30   b  to reach a lower surface of the upper form  30   a.  At this point a portion of the sound transmitted from the transmitter  32   a  is reflected from the lower surface of the upper form  30  and attenuates sharply as a result, and in that attenuated state the sound passes through the upper form  30   a  and is further attenuated thereby, attaining a level indicated as S 3  in FIG. 5. As a result, the sound reaches the receiver  32   b  in a greatly attenuated state indicated in FIG. 8A as W 4 .  
         [0079]    Accordingly, the attenuation W 4  at the portion of the double-fed forms where the air gap C is formed is much greater than either the attenuations W 1 , W 2  obtained with mere single forms  30  having slight variations in thickness and surface characteristics or the attenuation W 3  obtained when two forms  30  are double-fed without any air gap formed in between. Consequently, a state in which sheets are double fed can be distinguished with a high degree of accuracy from a state in which single sheets are fed properly. As a result, by setting an appropriate threshold level L 1  for distinguishing the former state from the latter (appropriate insofar as the effects of external disturbances on the unit are taken into account), whenever the attenuation W exceeds the threshold level L 1  (in terms of FIG. 8, whenever the transmitted sound volume as an absolute value drops below the threshold value L 1 , a double feed can be detected. Upon detection a detection signal can be used to trigger an alarm indicator and to stop the optical form reader  12  as appropriate.  
         [0080]    In the case described above, in place of the output voltage signals of the ultrasonic sensor  32  (the output voltage signals corresponding to the transmitted sound volume), the output voltage signals can be inverted (as shown in FIG. 8B) by an inversion amplifying circuit, the inverted voltage signal can be held at a peak value for a predetermined time period by a peak hold circuit, and when that peak hold value exceeds a slice level L 2  as shown in FIG. 8C, a comparator output can be turned ON in a FIG. 8D. In so doing, instantaneous changes in the output signals of the ultrasonic sensor  32  generated by a double feed can be detected quickly and accurately.  
         [0081]    Next, a description will be given of a double feed detection device  10   b  according to a second embodiment of the present invention, with respect to FIG. 9.  
         [0082]    [0082]FIG. 9 is a lateral view of the double feed detection device according to the second embodiment of the present invention.  
         [0083]    The double feed detection device  10   b  comprises a transmitter  32   a  and a receiver  32   b  (together comprising an ultrasonic sensor  32 ) provided above and below a transport path surface of the transport path  22  on the downstream side (X 1  side) of the form separator  18 . Additionally, a pad-like, curved panel member  50  and a roller  52  are disposed opposite each other above and below the transport path  22  downstream of the ultrasonic sensor  32 .  
         [0084]    According to the double feed detection device  10   b  described above, the pad-like member  50  performs the same role as the roller  36   a  of the first embodiment, and so the same effects as with the first embodiment of the present invention can be obtained. In this case, the use of the pad-like member  50  in place of the roller  36   a  simplifies the design of the device. It should be noted that when this pad-like member is made of an electrically conductive material it is possible to remove any static electric charge residing on the forms  30  during their transport, which removal is desirable.  
         [0085]    Next, a description will be given of a double feed detection device  10   c  according to a third embodiment of the present invention, with respect to FIG. 10.  
         [0086]    [0086]FIG. 10 is a plan view of the double feed detection device according to the third embodiment of the present invention.  
         [0087]    The double feed detection device  10   c  has pairs of rollers  54   a,    54   b  and  56   a,    56   b  disposed opposite each other along both sides as well as above and below the transport path  22  at a position downstream (in the X 1  direction) of the form separator  18 . The transmitter  32   a  and the receiver  32   b  of the ultrasonic sensor  32  are positioned downstream in the X 1  direction of the pair of paired rollers.  
         [0088]    The rollers  54   a,    56   a  are arranged so that their rotation shafts are at right angles to the transport direction (the Y direction). By contrast, however, rollers  54   b  and  56   b  are positioned so that their rotation shafts are offset with respect to the centerlines of the rollers  54   a  and  56   a  by a slight angle indicated in FIG. 10 as θ. It should be noted that in FIG. 10 angle θ has been exaggerated for purposes of illustration only, and that in actuality the angle is small and set experimentally, that is, is varied according to the quality of the forms  30 . The rollers  54   a  and  56   a  are provided on the same shaft and form the drive side, that is, the transport part. The rollers  54   b,    56   b  press the lateral edges of the form  30  inward toward a center of the form  30 .  
         [0089]    When the forms  30  are double fed, the rollers  54   a,    54   b  and  56   a,    56   b  work to press the lateral edges of the forms  30  toward the center of the forms  30 , such that the bottom form  30   b  acquires a bulge, thereby opening a gap between it and the top form  30   a  in which no bulge is formed.  
         [0090]    As described above, the double feed detection device  10   c  according to the third embodiment of the present invention also achieves the same effect as the double feed detection device  10   a  according to the first embodiment of the present invention as described above.  
         [0091]    It will be appreciated by those of skill in the art that the present invention is not limited to the detection of double feeds of paper as described above but can be applied to virtually any sheet-shaped detection object and thus can include paper, plastic and metal detection objects.  
         [0092]    Moreover, it should be noted that, although the embodiments described above make reference to an optical form reader, the present invention is not limited to such embodiments but can be adapted to any processing unit accommodating sheet-like detection objects.  
         [0093]    The above description is provided in order to enable any person skilled in the art to make and use the invention and sets forth the best mode contemplated by the inventors of carrying out the invention.  
         [0094]    The present invention is not limited to the specifically disclosed embodiments, and variations and modifications may be made without departing from the scope and spirit of the present invention.  
         [0095]    The present application is based on Japanese Priority Application No. 2000-150335, filed on May 22, 2000, the entire contents of which are hereby incorporated by reference.