Title:
Double feed detection method and device
Kind Code:
A1


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.



Inventors:
Minoru, Wada (Kawasaki, JP)
Satoh, Shinichi (Kawasaki, JP)
Application Number:
09/758147
Publication Date:
11/22/2001
Filing Date:
01/12/2001
Assignee:
MINORU WADA
SATOH SHINICHI
Primary Class:
Other Classes:
271/263
International Classes:
G01B17/02; B65H5/06; B65H7/12; (IPC1-7): B65H7/12
View Patent Images:



Primary Examiner:
BOLLINGER, DAVID H
Attorney, Agent or Firm:
KRATZ, QUINTOS & HANSON, LLP (WASHINGTON, DC, US)
Claims:

What is claimed is:



1. 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.

2. 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: 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 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.

3. The double feed detection device as claimed in claim 2, wherein the gap-forming mechanism comprises: a transport part that sandwiches the sheet-like detection object at a trailing end of the transport path in a direction of transport of the sheet-like detection object by a transporting mechanism along the transport path; and a brake part, the brake part being a resistive element that reduces a speed with which the sheet-like detection object is transported by the transporting mechanism, the brake part generating a bulge, in the direction of transport of the sheet-like detection object along the transport path, in the sheet-like detection object that contacts the brake part.

4. The double feed detection device as claimed in claim 2, wherein the gap-forming mechanism is provided at a trailing end of the transport path in the direction of transport of the sheet-like detection object by the transporting mechanism along the transport path and the gap-forming mechanism includes a transport part that sandwiches the sheet-like detection object at both lateral edges, the double feed detection device further comprising a part that presses the lateral edges of the sheet-like detection object inward toward a center of the sheet-like detection object, such that a width of a gap formed between double fed sheet-like detection objects is at a right angle to the direction of transport of the sheet-like detection object.

5. The double feed detection device as claimed in claim 3, wherein the brake part and the transport part comprise at least one pair of rollers that sandwich the sheet-like detection object between them, the roller that constitutes the brake part having a rotation speed that is slower than a rotation speed of the roller that constitutes the transport part.

6. The double feed detection device as claimed in claim 3, wherein the brake part is a pad-like member that scrapes one surface of the sheet-like detection object.

7. An optical form reader comprising: a scanner including a part that optically reads information stored on a form; and 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, the double feed detection device comprising: an ultrasonic transmitter and an ultrasonic receiver positioned so as to sandwich a transport path along which the forms are transported; and a gap-forming mechanism that forms a gap between the forms where the forms overlap when a double feed of the forms occurs.

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] 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 1a is provided with a mesh-like support stand 2 through which ultrasonic waves pass. Sheets of paper 3a 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 4a and ultrasonic receiver element 4b, disposed opposite each other. The ultrasonic receiver element 4b is mounted on an internal holder 5b of a cooling jacket case 5a, the internal holder 5b having a skirt 5c on a bottom part thereof.

[0009] An annular nozzle 6 is provided in a space between the skirt 5c and a jacket outer wall 5d, such as to allow a flow of air, that is, an air curtain 7, to be generated so as to press the paper 3a 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 3a 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 1b has an ultrasonic sound transmitter 8a that transmits ultrasonic sounds of a predetermined intensity and an ultrasonic sound receiver 8b disposed opposite each other and sandwiching the transport path through which the sheet-like detection object 3b passes. The intensity of the ultrasonic sounds detected by the ultrasonic sound receiver 8b 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 3b 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 3b when two such sheet-like detection objects 3b pass between the ultrasonic sound transmitter 8a and the ultrasonic sound receiver 8b is virtually constant regardless of the material or the thickness of the detection object 3b, 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] FIG. 1 is a lateral cross-sectional view of a conventional double feed detection device;

[0031] FIG. 2 is a structural diagram of another conventional double feed detection device;

[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] FIG. 4 is a flow chart illustrating an operation of the optical scanner shown in FIG. 3;

[0034] FIG. 5 is a side view of a double feed detection device according to a first embodiment of the present invention;

[0035] FIG. 6 is a circuit structure diagram of the double feed detection device shown in FIG. 5;

[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] FIGS. 8A, 8B, 8C and 8D 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] FIG. 9 is a lateral view of a double feed detection device according to a second embodiment of the present invention; and

[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] 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] 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 X1 direction to the form separator 18 in a step S1.

[0050] If for some reason the forms 30 are double fed, then they are separated by the form separator 18 in a step S2. 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 S3, and flashes an alarm indicator and causes the optical form reader to stop in an stop or alarm step S4.

[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 S5, 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] 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 10a according to the first embodiment of the present invention comprises an ultrasonic sensor 32 transmitter 32a and an ultrasonic sensor 32 receiver 32b disposed opposite each other so as to sandwich the transport path 22 on the X1 side of the form separator 18. Additionally, paired rollers 34a, 34b and 36a, 36b are provided at upstream and downstream sides, respectively, of the ultrasonic sensor 32.

[0057] The ultrasonic sensor 32 transmitter 32a is a transmitter, and the receiver 32b is a microphone. Ultrasonic sound waves of a predetermined intensity are generated from the transmitter 32a, 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 32b.

[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 32b 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] 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 34a, 34b and rollers 36a, 36b 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 48a, and the gear 48a is linked in order to gears 48b, 48c and 48d in one direction and, in the other direction, is linked in order to gears 48e, 48f, 48g 48h, 48i, and 48j. Gear 48h and roller 34a, and gear 48b and roller 34b, and gear 48j and roller 36a, and gear 48d and roller 36b are each provided on the same shaft. In this case, the gears 48h, 48b and 48d have the same diameter, with the gear 48j having a diameter that is slightly larger than that of gears 48h, 48b and 48d.

[0062] The rollers 34a, 34b and the rollers 36a, 36b are in this case all of the same diameter. Therefore, when the gears are rotated by the motor 46, only the roller 36a, which is directly linked to the gear 48j (the gear 48j having the slowest rotation speed), rotates more slowly than the other rollers 34a, 34b and 36b.

[0063] A description will now be given of the double feed detection device 10a 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 X1 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 10a.

[0067] In this case, the coefficient of friction between the form 30 and the rollers 36a and 36b is set to be larger than the coefficient of friction between the double-fed forms 30. As a result, a form 30b that contacts the roller 36b (the bottom form shown in FIG. 4) starts to slip with respect to the form 30a on top of the bottom form 30b, such that the rotation of the roller 36b transports the bottom form 30b at an ordinary transport speed in the X1 direction onward to a next processing stage. At the same time, the top from 30a, which contacts the roller 36a, is sent onward by the roller 36a in the X1 direction to the next processing stage at a transport speed slightly less than that of the roller 36b.

[0068] Accordingly, the top form 30a, which is sent onward at a transport speed that is slower than that of the bottom form 30b, gradually forms a bulge A upstream of the roller 36a, that is, on an X2 side. If, for example, the ratio of the rotation speed of the roller 36a to the roller 36b is 30:29, then when the lower form 30b has advanced 30 mm the upper form 30a 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 30a and 30b. Thus the rollers 34a, 34b, 36a and 36b function as a gap-forming mechanism, forming a gap between double-fed forms 30a and 30b. In other words, the roller 34a acts as a brake and the roller 34b 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 32a and 34b is short, the rollers 32a and 34b can be eliminated so that just the rollers 36a and 36b 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 8C.

[0070] FIGS. 8A, 8B, 8C and 8D 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] 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 t1-t2 (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 W1. (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 t1 is taken as the reference acoustic level).

[0073] During the time t2 to t3, that is, during the time required for a succeeding N form 30 to reach the detection point at time t3, the transmitted sound volume hardly attenuates at all.

[0074] When the N form 30 passes through the ultrasonic sensor 32 detection point during time t3-t4, the amount of sound passing through the N form 30 shows an attenuation of W2 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 t5, 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 t5-t6 in FIG. 8A, causing an attenuation in transmitted sound volume shown as W3 in FIG. 8A.

[0077] As can be appreciated from FIG. 8A, the attenuation W3 obtained at time interval t5-t6, 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 W1, W2, obtained when N-1 and N forms 30 pass the detection point as single sheets.

[0078] By contrast, when the rollers 36a, 36b that function as gap-forming means form a gap C between the upper and lower double-fed sheets 30a, 30b, and when that portion of the double-fed forms 30a, 30b in which the gap C is formed passes the detection point at time intervals t6-t7 and again t8-t9, the sound transmitted from the transmitter 32a and shown as level S1 in FIG. 5 passes through the lower sheet 30b to become a level S2 as shown in FIG. 5, further passing through the air gap formed between the upper and lower forms 30a, 30b to reach a lower surface of the upper form 30a. At this point a portion of the sound transmitted from the transmitter 32a 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 30a and is further attenuated thereby, attaining a level indicated as S3 in FIG. 5. As a result, the sound reaches the receiver 32b in a greatly attenuated state indicated in FIG. 8A as W4.

[0079] Accordingly, the attenuation W4 at the portion of the double-fed forms where the air gap C is formed is much greater than either the attenuations W1, W2 obtained with mere single forms 30 having slight variations in thickness and surface characteristics or the attenuation W3 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 L1 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 L1 (in terms of FIG. 8, whenever the transmitted sound volume as an absolute value drops below the threshold value L1, 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 L2 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 10b according to a second embodiment of the present invention, with respect to FIG. 9.

[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 10b comprises a transmitter 32a and a receiver 32b (together comprising an ultrasonic sensor 32) provided above and below a transport path surface of the transport path 22 on the downstream side (X1 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 10b described above, the pad-like member 50 performs the same role as the roller 36a 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 36a 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 10c according to a third embodiment of the present invention, with respect to FIG. 10.

[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 10c has pairs of rollers 54a, 54b and 56a, 56b disposed opposite each other along both sides as well as above and below the transport path 22 at a position downstream (in the X1 direction) of the form separator 18. The transmitter 32a and the receiver 32b of the ultrasonic sensor 32 are positioned downstream in the X1 direction of the pair of paired rollers.

[0088] The rollers 54a, 56a are arranged so that their rotation shafts are at right angles to the transport direction (the Y direction). By contrast, however, rollers 54b and 56b are positioned so that their rotation shafts are offset with respect to the centerlines of the rollers 54a and 56a 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 54a and 56a are provided on the same shaft and form the drive side, that is, the transport part. The rollers 54b, 56b 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 54a, 54b and 56a, 56b work to press the lateral edges of the forms 30 toward the center of the forms 30, such that the bottom form 30b acquires a bulge, thereby opening a gap between it and the top form 30a in which no bulge is formed.

[0090] As described above, the double feed detection device 10c according to the third embodiment of the present invention also achieves the same effect as the double feed detection device 10a 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.