Patent Publication Number: US-6698753-B2

Title: Device for and method of detecting an overlap in paper being transported

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a device for and a method of detecting an overlap in paper being transported. More particularly, this invention relates to such a device and a method capable of dependably informing the user whether or not the detected overlap is of a kind to be avoided or a kind that may be ignored. 
     When a plurality of sheets of paper to be scanned individually are simultaneously transported in an overlapped condition even partially, it is not possible to carry out an accurate scan. For this reason, many scanners of this type are provided with an overlap detector, or a detector for detecting an overlapped condition of paper sheets being transported such that the transportation of the paper sheets may be stopped temporarily. In other words, scanning by such a scanner is interrupted when the overlap detector detects an overlapped condition. 
     The scanning operation of a scanner is interrupted also when the overlap detector detects an overlapped condition erroneously, forcing the user to go through the troublesome process of investigating the cause of the interruption and restarting the scanning. Another problem with such a prior art overlap detector is that it always concludes that there is an overlap of the kind for which the transportation of paper should be interrupted whenever two sheets of paper are found to be overlapping. In other words, doubly folded sheets and two sheets of intentionally overlapped paper are both treated as representing an overlapped condition for which the transportation of the paper should be interrupted. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of this invention to provide a device for and a method of accurately detecting an overlap in paper being transported such that doubly folded paper and two sheets of paper that are intentionally overlapped may be distinguished. 
     An overlap detecting device according to this invention may be characterized as comprising a detecting means for detecting an overlapped condition in a paper sheet being transported on its transportation path, a data generating device for generating a specified data item on the overlap detected by the detecting means, a level selector for selecting one of a plurality of preliminarily defined levels corresponding to the specified data item generated by the data generating device, and an output device for outputting the selected level by the level selector. Thus, as an overlapped condition of a paper sheet being transported on a path is detected, a specified data item about the overlap is generated, its level is selected from a plurality of preliminarily defined levels and the selected level is outputted to be referenced by the user. The user can thereby determine whether the overlap was of an intentional or unintentional kind. The data generating device and the level selector may each comprise a CPU. 
     For the detection of the overlap, use may be made of an ultrasonic wave generator for generating ultrasonic waves to be made incident on the path of transportation of the paper sheet, an ultrasonic receiver for receiving the ultrasonic waves generated by the ultrasonic wave generator and a phase difference detector which may comprise a CPU for detecting a phase difference between the ultrasonic waves received by the ultrasonic wave receiver and a predetermined standard phase. An overlap in a paper sheet being transported on the path can be detected from the phase difference detected by the phase difference detector. 
     The aforementioned data item serves to indicate a possibility of occurrence of the overlap, and the preliminarily defined levels include a first level indicating that the overlap is certainly taking place and a second level indicating that there is a possibility that the overlap is taking place. Thus, if the first level is outputted during a scanning operation, the scanning is interrupted but the scanning can be continued if the output is the second level. In this manner, interruption of the scanning due to an erroneous detection of an overlap can be restrained. 
     The aforementioned data item may include a cumulative number of times the overlap has been detected by the overlap detector and the level selector may include a first level selector and a second level selector, the first level selector selecting the first level if the cumulative number is equal to or greater than a predefined first threshold value, the second level selector selecting the second level if the cumulative number is equal to or greater than a predefined second threshold value which is smaller than the first threshold value. The first and second level selectors may each comprise a CPU. In this manner, the level of possibility for the occurrence of an overlap can be dependably determined, and these levels can be freely adjusted by the user or by the maker by merely changing the values of the first and second threshold values. 
     The aforementioned data generating device may reset the cumulative number of times to zero if the paper sheet is on its path of transportation but the overlap has not been detected by the overlap detector. In this manner, the levels for determining the possibility of an overlap can be set more reliably. 
     There may also be provided a judging device for judging that there is no possibility that an unintentional overlap has occurred if the overlap is detected by the overlap detector over an entire detection range of the paper sheet from its front edge to its back edge. In other words, a doubly folded paper sheet or two intentionally overlapped sheets of paper are not considered to be an unwanted overlap. Thus, a scanner provided with a detecting device of this invention can operate on doubly folded paper sheets and pairs of intentionally overlapped paper sheets. Such a judging device may comprise a CPU which may not be the same as any of the CPUs referred to above but may comprise a CPU of a multi-purpose personal computer. 
     An overlap detection method of this invention may be characterized as comprising the steps of generating ultrasonic waves to be made incident on the path of transportation of the paper sheet, receiving the ultrasonic waves made incident on the path, detecting a phase difference between the received ultrasonic waves and a predetermined standard phase, detecting an overlap in a paper sheet being transported on the path on the basis of the detected phase difference, generating a specified data item on the detected overlap, selecting one of a plurality of preliminarily defined levels corresponding to the specified data item, outputting the selected level, and judging that there is no possibility of an unintentional overlap if the overlap is detected over an entire detection range of the paper sheet from its front edge to its back edge. When an overlap is detected by a method of this invention, a specified data item is generated on the detected overlap, a level for the specified data item is selected from a plurality of preliminarily defined levels and the selected level is outputted. Thus, the user can easily judge whether or not the detected overlap is an unwanted overlap. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a device for detecting an overlap embodying this invention. 
     FIG. 2 is a graph for showing the principle of detecting an overlap by the phase of ultrasonic waves. 
     FIG. 3 is a block diagram of a host PC of the overlap detecting device of FIG.  1 . 
     FIG. 4 is a flowchart for the processing by the overlap detection unit of the device of FIG.  1 . 
     FIG. 5 is another flowchart for the processing by the overlap detection unit of the device of FIG.  1 . 
     FIG. 6 is a timing chart for the transmission timing of paper detection signal, possibility message signal and overlap message signal. 
     FIG. 7 is a flowchart for the processing by the host PC of the device of FIG.  1 . 
     FIGS. 8,  9  and  10  are examples of display when it is judged that there is a possibility of an overlap. 
     FIG. 11 is a flowchart for the processing by the overlap detection unit of FIG. 1 according to a second embodiment. 
     FIG. 12 is another flowchart for the processing by the overlap detection unit of FIG. 1 according to the second embodiment. 
     FIG. 13 is a diagram for showing the correspondence between the paper position and the overlap data memory address. 
     FIG. 14 is a flowchart for the processing by the host PC of the device of FIG. 1 according to the second embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1 shows the structure of an overlap detector  1  embodying this invention, comprising a host personal computer (PC)  11  for setting various parameters and thereby controlling an overlap detection unit  12 . The overlap detection unit  12  includes a CPU  21  which serves to control the operations of each component of the overlap detection unit  12  through a processing unit  93  on the basis of various signals received from the host PC  11 . The CPU  21  also controls the overall operations of the overlap detection unit  12  through the processing unit  93  according to a program stored in a memory device  22 . The memory device  22  additionally stores various data necessary for the CPU  21  in carrying out various operations. The CPU  21  also serves to transmit to the host PC  11  various data stored in the memory device  22  as well as signals supplied from the processing unit  93  (such as a paper detection signal  142 , a “possibility message signal”  142  and an “overlap message signal”  143  to be explained below), whenever necessary. 
     The overlap detection unit  12  has a motor driver  24  for driving and thereby causing a motor  25  to rotate. The rotary motion of the motor  25  is communicated to a roller  27  through a belt  26 , and the rotary motion of the roller  27  is further communicated to another roller  28  through another belt  29 . Rollers  30  and  31  are pressed respectively against the rollers  27  and  28  such that a sheet of paper  41  clamped between the rollers  27  and  30  is transported on a paper feeding plate  32  from the right to the left with reference to the figure, and clamped between the rollers  28  and  31  to be transported further to the left. 
     The image on the paper sheet  41  thus transported on the paper feeding plate  32  is scanned by a scanner  3  on the basis of a control by the host PC  11 . The scanned image data are supplied from-the scanner  3  to the host PC  11 . 
     Numeral  61  indicates an ultrasonic transmitter controlled by an oscillation amplifier  73  to transmit ultrasonic waves onto the transportation path of the paper sheet  41 . Ultrasonic waves transmitted from the transmitter  61  and passing through the paper sheet  41  as well as waves passing through a hole  32 A formed through the paper feeding plate  32  are received by an ultrasonic receiver  62 . The receiver  62  also serves to output a reception signal to an amplifier  74 . 
     A clock signal generated by an oscillator  72  is supplied to a control block  72  which carries out various processes in synchronism with this clock signal such as controlling the oscillation amplifier  73  such that the ultrasonic transmitter  61  is driven thereby and will generate ultrasonic waves. 
     The output from the oscillation amplifier  73  is transmitted to a filter  91  of the control block  71  through an AD converter  81  for monitoring. The filter  91  serves to eliminate the high-frequency noise components from the inputted signal and to output the signal to a peak detector unit  92 . The peak detector unit  92  serves to detect a peak value from the inputted signal and outputs the detected peak value to the processing unit  93 . 
     The amplifier  74  amplifies the output from the receiver  62  and transmits the amplified output to a level judging unit  75  for evaluating the level of the signal inputted from the amplifier  74 . If this level is above a threshold value, it is judged that there is no paper sheet  41 . If this level is below the threshold value, it is judged that the paper sheet  41  is present. If it is judged that the paper sheet  41  is present, a detection signal (the “paper detection signal”) is outputted to the processing unit  93 . The paper detection signal is also supplied to the host PC  11  through the CPU  21 . At the same time, analog switches  78  and  79  are respectively switched on and off by the level judging unit  75  and the output from the amplifier  74  is inputted to an AD converter  80 . 
     If it is judged that the paper sheet  41  is absent, on the other hand, the analog switches  78  and  79  are respectively switched off and on by the level judging unit  75  and the output from the amplifier  74  is divided (and hence attenuated) by means of resistors  76  and  77  and then inputted to the AD converter  80 . 
     The high frequency noise components of the signal inputted from the AD converter  80  are eliminated by a filter  95  and the filtered signal is outputted to a peak detector unit  96  for detecting a peak value of the signal inputted from the filter  95  and outputting it to the processing unit  93 . 
     Clock signals supplied from the oscillator  72  are counted by a loop counter  94  and the counted values are outputted to the processing unit  93 . The number of times the phase difference of the received signal has exceeded a threshold value (AZ) (hereinafter referred to as the “overlap frequency J” where J is an integer) is counted by a judgement counter  97 . The number of sampling is counted by a data number counter  98 . 
     If the frequency of the signal outputted to the ultrasonic wave transmitter  61  from the amplifier  73  is f, the frequency of the clock signals outputted from the oscillator  72  may be set equal to  360   f . The AD converters  80  and  81  carry out sampling operations at this frequency ( 360   f ) and the loop counter  94  counts the clock signals also at this frequency ( 360   f ). 
     FIG. 2 is referenced next to explain the principle of detecting an “overlap phase” on the basis of the phase of the received signal of ultrasonic waves. 
     FIG. 2A shows the level and the phase of the ultrasonic waves transmitted from the transmitter  61  (or the signal for controlling the transmitter  61  by the amplifier  73 ). When the transmitter  61  transmits an ultrasonic wave with such a phase on the basis of a control signal from the amplifier  73 , the receiver  62 , upon receiving the wave, outputs a reception signal as shown in FIG. 2B or  2 C to the amplifier  74 . 
     FIG. 2B shows the level and the phase of the reception signal when the paper sheet  41  is not present on the path of its transportation (on the propagation path of the ultrasonic waves), and FIG. 2C shows those when the paper sheet  41  is present. They clearly show that the signal level is higher when the paper sheet  41  is absent (FIG. 2B) than when the paper sheet  41  is present (FIG.  2 C). It is reminded that the signal level is shown in units of 200 mV/div in FIGS. 2A and 2B but in units of 20 mV/div in FIG.  2 C. 
     When the paper sheet  41  is not present, the phase lag of the received signal with respect to the transmitted signal is θ 1 , that is, the phase difference between the peak P A  of the transmitted signal shown in FIG.  2 A and the peak P B  of the received signal shown in FIG. 2B is θ 1 . When a single sheet of paper  41  is present, by contrast, the phase lag of the received signal (FIG. 2C) with respect to the transmitted signal (FIG. 2A) becomes θ 2 , that is, the phase difference between the peak P C  of the received wave shown in FIG.  2 C and the peak P A  of the transmitted wave shown in FIG. 2A becomes θ 2 . 
     These phase differences θ 1  and θ 2  have different values. Variations in the phase difference θ 2  when a single sheet of paper  41  is present are relatively small, being within the range of ±ΔZ with respect to the phase difference θ 2 . 
     When there is an overlap, however, the phase difference θ in such a situation is not within the range of θ 2 ±ΔZ but falls outside of this range. This makes it possible to determine whether or not there is an overlap by examining whether or not the phase difference of the received signal is within a standard range (θ 2 ±ΔZ). 
     In what follows, the phase difference θ of the reception signal will be referred to as the “reception phase difference” and the phase difference determined not to be within the range of θ 2 ±ΔZ, that is, the phase difference determined to be indicating an overlap will be referred to as the “overlap phase difference.” 
     Since the determination whether a phase difference is an overlap phase difference or not is to be made by using a relative change in the phase of the received wave, the starting point for the measurement of the phase of the received wave need not be selected as in the illustrated example. If the received wave is of a standard waveform with the same frequency as the transmitted wave, any point may be selected if the relative phase difference of the received wave can be obtained. For example, a standard (reference) waveform such as shown in FIG. 2D having the same frequency as the transmitted wave shown in FIG.  2 A and the relative phase θ 2 ′ of the received wave (FIG. 2C) may be determined from its rising edge serving as the base point. In such a case, the phase θ 1 ′ at the time of no paper sheet is also measured from the same base point, that is, the rising edge of the reference waveform. In the example, the determination on an overlap phase difference is made on this principle. 
     FIG. 3 shows an example of the structure of the host PC  11 . Numeral  111  therein indicates a CPU adapted to carry out various operations according to programs stored in an ROM  112  or programs loaded from a memory device  118  to a RAM  113 . Various data necessary for the CPU  111  in carrying out its various operations are also conveniently stored in the RAM  113 . 
     The CPU  111 , the ROM  112  and the RAM  113  are mutually connected through a bus  114 , to which an I/O interface  115  is also connected. An input device  116  comprising a keyboard and a mouse, an output device  117  comprising a CRT display, the memory device  118  comprising a hard disk, and a communication device  119  comprising a modem and terminal adapters are connected to the I/O interface  115 . The communication device  119  carries out network communications inclusive of internet communications. 
     If necessary, a driver  120  may be connected to the I/O interface  115  such that a magnetic disk  131 , an optical disk  132 , a photo-electromagnetic disk  133 , or a semiconductor memory  134  may be attached and a computer program read out from them may be installed in the memory device  118 . 
     When a series of operations are executed by software, a program comprising this software is installed from a network or a memory medium onto a computer installed in dedicated hardware or a multi-purpose personal computer having various programs installed so as to be able to carry out various functions. 
     The memory medium may comprise, as shown in FIG. 3, a magnetic disk  131  (inclusive of a floppy disk), an optical disk  132  (inclusive of a CD ROM and a DVD), an photo-electromagnetic disk  133  (inclusive of mini-Disks) and packaged media including a semiconductor memory  134  which stores programs and may be distributed to the user for supplying programs, apart from the apparatus itself. It may also include the ROM  112  and a hard disk included in the memory device  118  storing programs which are supplied to the user in the form of already being installed in the apparatus. 
     The aforementioned scanner  3  and the overlap detection unit  12  are also connected to the I/O interface  115 . 
     Thus, a memory medium storing programs to be executed by the overlap detection unit  12  may be connected to the driver  120  such that the programs read therefrom may be supplied to the overlap detection unit  12  through the I/O interface, whenever necessary, and further installed in the memory device  22  through the CPU  21  inside the overlap detection unit  12 . 
     The flowchart of FIG. 4 is referenced next to explain an example of processes carried out by the overlap detection unit  12 . First, the CPU  21  resets the overlap frequency J of the judgment counter  97  equal to zero through the processing unit  93  (Step S 11 ). Next, the CPU  21  checks whether or not the paper sheet  41  has been detected (Step S 12 ). Explained more in detail, if the paper detection signal has been supplied from the level judging unit  75  through the processing unit  93 , the CPU  21  concludes that the paper sheet  41  has been received by inputting and recognizing this signal. If otherwise (that is, if no detection signal is supplied), it concludes that the paper sheet  41  has not been detected. 
     If it is determined that the paper sheet  41  has not been detected (NO in Step S 12 ), the CPU  21  waits until it is detected. When the paper sheet  41  is detected (YES in Step S 12 ), the reception phase difference is obtained (Step S 13 ) and the paper detection signal, which has been supplied from the level judging unit  75  through the processing unit  93 , is transmitted to the host PC  11 . Next, the CPU  21  determines whether the reception phase difference is an overlap phase difference (Step S 14 ). Details of Steps  13  and  14  are the same as already explained above with reference to FIG.  2 . 
     If it is determined not to be an overlap phase difference (NO in Step S 14 ), the CPU  21  checks whether or not the paper sheet  41  has been detected (Step S 20 ). This is similar to Step S 12  but is different in that Step S 132  is for determining whether or not the paper sheet  41  has been brought onto its transportation path, or whether or not its transportation has been started, while Step S 20  is for determining whether or not the paper sheet  41  has been transported out of the path, or whether or not its transportation has been completed. 
     If it is determined that the paper sheet  41  is not detected (NO in Step S 20 ), the CPU  21  ends this processing. If it is determined that the paper sheet  41  is detected, or that the paper sheet  41  is still being transported (YES in Step S 20 ), the CPU  21  returns to Step S 13  and the subsequent Steps are repeated. 
     If it is determined to be an overlap phase difference (YES in Step S 14 ), the value of the overlap frequency J is incremented by 1 (Step S 15 ). If the incremented overlap frequency J is equal to or greater than a predefined threshold value K1 greater than 1 (YES in Step S 16 ), the CPU  21  generates a signal indicating a “possibility of an overlap” and outputs this signal to the host PC  11  (Step S 17 ). If the incremented overlap frequency J is less than the threshold value K1 (NO in Step S 116 ) or if the process of Step S 17  has been completed, the CPU  21  checks whether or not the incremented overlap frequency J is equal to or greater than a larger threshold value K2 (Step S 18 ). If the incremented overlap frequency J is determined to be equal to or greater than this threshold value K2 (YES in Step S 18 ), the CPU  21  generates a signal indicating an “overlap” and outputs it to the host PC  11  (Step S 19 ). If the incremented overlap frequency J is determined to be less than K2 (NO in Step S 18 ) or if the process of Step S 19  has been completed, the CPU  21  checks whether or not the paper sheet  41  is detected (Step S 20 ), as explained above. 
     In summary, the overlap detection unit  12  looks for an overlapped condition at a specified timing of sampling as long as the paper sheet  41  is being transported, or as long as the paper sheet  41  is being detected, to count the overlap frequency J and judges the possibility of occurrence of an overlap on the basis of the value of this overlap frequency J, the possibility of occurrence being outputted in two levels. If J is equal to or greater than the greater of two threshold values (K2), it is judged that there is certainly an overlap. If J is equal to or greater than the smaller of the two threshold values (K1) but smaller than the larger threshold value K2, it is judged that there is only a possibility of an overlap. 
     The invention does not impose any particular limitation on the manners in which Steps S 16 -S 19  should be carried out. The CPU  21  may carry out actual operations J≧K1 or J≧K2 or compare the overlap frequency J with a preliminarily prepared table on threshold values stored in the memory device  22 . 
     Although an example with two judgment levels has been shown above, neither is the number of levels intended to limit the scope of the invention. The number of the levels may be increased by introducing a corresponding number of threshold values. 
     FIG. 6 is a time chart for the transmission of signals corresponding to the judgments at these levels. When it is determined in Step S 12  that the paper sheet  41  has been detected, the overlap detection unit  12  transmits to the host PC  11  a paper detection signal  141 . If J is determined to be equal to or greater than K1 in Step S 16 , a “possibility message signal”  142  is outputted in Step S 17 . If it is determined that J is equal to or greater than K2 in Step S 18 , an “overlap message signal”  143  is outputted in Step S 19 . 
     Thus, when it is determined that there is a possibility of an overlap, the host PC  11  can continue the scanning operation of the scanner  3 . As a result, the stopping of the transportation of the paper sheet  41  due to an erroneous detection of an overlap can be controlled and the user can be freed from the work of investigating the cause of the interruption in the transportation. Since a “possibility message signal” is transmitted to the user, furthermore, the user can postpone the investigation of the possibility until the scanning operation is completed. 
     An advantage of using two threshold values is that the level judgment can be carried out easily and that the user can easily vary the levels by changing the values of K1 and K2. 
     FIG. 5 shows another example of processing by the overlap detection unit  12  of FIG.  1 . The flowchart of FIG. 5 is different from that in FIG. 4 only in that the overlap frequency J is reset to zero (Step S 41 ) when it is determined in Step S 14  that it is not a case of an overlap. This step is advantageous in that the possibility of an overlap can be more correctly judged and an erroneous detection of an overlap can be more reliably controlled. 
     FIG. 7 is referenced next to explain an example of processing by the host PC  11  corresponding to the processing of the overlap detection unit  12  according to FIG. 4 or  5 . 
     To start, the CPU  111  of the host PC  11  resets to zero the number P of paper sheets  41  sent onto the path of transportation (hereinafter referred to as the paper number, P being an integer). Explained more in detail, an area (referred to as the P-area) for storing the page number P is secured on the RAM  113  and numeral 0 is stored in the P-area as the initial value (Step S 51 ). Next, the CPU  111  sends off one sheet of paper  41  onto its path of transportation (or places it on the paper feeding plate  32 ) by generating a paper transportation signal and supplying it to the overlap detection unit  12  through the bus  114  and the I/O interface  115  (Step S 52 ). On the basis of this signal thus supplied, the CPU  21  of the overlap detection unit  12  controls its motor driver  24  through the processing unit  93  to rotate the motor  25  and to thereby transport the paper sheet  41 . 
     Next, the paper number P is incremented by 1 and the incremented page number P =P+ 1  is stored in the P-area (Step S 53 ). Next, the CPU  111  determines whether or not the overlap detection unit  12  has detected the paper sheet  41  by checking whether or not the aforementioned paper detection signal  141  has been transmitted from the overlap detection unit  12  (Step S 54 ). If it is determined that the overlap detection unit  12  has not detected the paper sheet  41  (NO in Step S 54 ), the CPU  111  waits until the detection is made. If it is determined that the overlap detection unit  12  has detected the paper sheet  41  (YES in Step S 54 ), the CPU  111  further undertakes to determine whether or not the overlap detection unit  12  has detected the paper sheet  41  (Step S 55 ). Explained more in detail, the detection in Step S 54  is for judging whether or not the paper sheet  41  has been brought onto its path of transportation, or whether or not its transportation has started, while the detection in Step S 55  is for judging whether or not the paper sheet  41  has been transported out from the its path of transportation, or whether or not its transportation has been completed. 
     If it is determined in Step S 55  that the overlap detection unit  12  has detected the paper  41 , or that the paper sheet  41  is being transported (YES in Step S 55 ), the CPU  111  continues to wait. If a “possibility message signal”  142  is received from the overlap detection unit  12  in Step S 17  of FIG. 4 or  5 , however, the CPU  111  stores a data item to this effect in a specified area (referred to as the “overlap data memory address”) on the RAM  113 . If an “overlap message signal”  142  is received from the overlap detection unit  12  in Step S 19  of FIG. 4 or  5 , the CPU  111  similarly stores a data item to this effect at the overlap data memory address. 
     If it is determined in Step S 55  that the paper sheet  41  is not being detected by the overlap detection unit  12 , or that the transportation of the paper sheet  41  has not been completed (NO in Step S 55 ), the CPU  111  reads out the data item stored at the overlap data memory address (Step S 56 ). If the overlap data memory address is found to store a data item indicative of an overlap (YES in Step S 57 ), the CPU  111  causes the transportation of the paper sheet  41  to be stopped (Step S 62 ). This process of stopping the transportation of the paper sheet  41  is carried out in the same way as the prior art process of stopping the motion of a paper sheet when an overlapped condition is encountered. Explained more in detail, the CPU  111  generates a stop signal and supplies to the overlap detection unit  12  through the bus  114  and the I/O interface  115 . Upon receiving this signal, the overlap detection unit  12  controls the motor driver  24  through the processing unit  93  to stop the rotary motion of the motor  25 , thereby stopping the motion of the paper sheet  41 . 
     If there is no data item indicative of an overlap (NO in Step S 57 ), the CPU  111  further examines whether or not a data item indicative of a possibility of an overlap is stored (Step S 58 ). If such a data item is found to be present (YES in Step S 58 ), the CPU  111  causes the page number P stored in the P-area to be stored in another area (referred to as the display area) on the RAM  113  (Step S 59 ). If it is determined that no data item indicative of a possibility of an overlap is present (NO in Step S 58 ), or if Step  59  has been completed, the CPU  111  checks whether or not all of the paper sheets  41  have been transported (Step S 60 ). If not all of the paper sheets  41  have been transported (NO in Step S 60 ), the CPU  111  returns to Step S 52  and repeats the subsequent Steps, causing the next one of the paper sheets  41  to be sent onto the path of transportation and checking whether or not this sheet is overlapped or there is such a possibility. 
     If it is determined that the transportation of all of the paper sheets  41  has been completed (YES in Step S 60 ), the CPU  111  causes the page number P stored in Step S 59 , or data corresponding to the page number P with a possibility message signal to be displayed on the output device (CRT)  117 . 
     The invention does not impose any particular limitation on the data to be thus displayed on the output device (CRT)  117  as long as the user can recognize the page number P corresponding to the possibility of an overlap. 
     Suppose now that six sheets of paper have been scanned. FIG. 8 is an example of display that may be made accordingly. Not only are images  151 - 1 - 151 - 6  displayed, there is also a message  152 , indicating any possibility of an overlap, including the corresponding page number P with the possibility. 
     Alternatively, as shown in FIG. 9, the display may be made such that the page with the possibility of an overlap (indicated by Numeral  161 ) may be shown in a different color so as to be easily distinguished from normally scanned images  151 - 1 - 151 - 4  and  151 - 6 . As a further alternative, as shown in FIG. 10, a special symbol such as a circle  171  may be displayed on the image with the stored page number P. 
     Next, the flowchart of FIG. 11 is referenced to explain the processing by the overlap detection unit  12  of FIG. 1 according to a second embodiment of the invention. 
     In an initialization step (Step S 71 ), the CPU  21  resets the overlap frequency J of the judgment counter  97  to zero through the processing unit  93  and resets to zero the value inside the area (referred to as the “paper position area”) on the memory device  22  for storing the paper position K. The CPU  21  also generates an initialization signal and supplies it to the host PC  11  to thereby set each value of the overlap data memory address on the RAM  113  of the host PC  11 . 
     Next, the CPU  21  checks whether or not a paper sheet  41  has been detected (Step S 72 ). If no paper sheet  41  has been detected (NO in Step S 72 ), the CPU  21  continues to wait. If it is determined that a paper sheet  41  has been detected (YES in Step S 72 ), a reception phase difference is obtained (Step S 73 ) and the paper detection signal supplied from the level judging unit  75  through the processing unit  93  is outputted to the host PC  11 . Next, the CPU  21  determines whether or not the obtained reception phase difference is an overlap phase difference (Step S 73 ). 
     If it is determined that it is an overlap phase difference (YES in Step S 73 ), the judgment counter  97  increments its overlap frequency J by 1, or J=J+1 (Step S 75 ). The processing of Steps S 72 -S 75  is the same as that of Steps S 12 -S 15  of FIG.  4  and hence will not be described repetitiously. 
     If the incremented overlap frequency J is equal to or greater than a specified threshold value K1 (YES in Step S 76 ), the CPU  21  generates a signal corresponding to a “possibility of an overlap” (Step S 77 ) and outputs it to the host PC  11 . Upon receiving this signal, the host PC  11  stores a value corresponding to the “possibility of an overlap” such as “1” at a corresponding address MA K  within its overlap data memory address  183 . 
     If the incremented overlap frequency J is determined to be less than K1 (NO in Step S 76 ), or if the process of Step S 77  is completed, the CPU  21  determines whether or not the overlap frequency J is greater or less than a larger threshold value K2 (Step S 78 ). If the overlap frequency J is equal to or greater than K2 (YES in Step S 78 ), the CPU  21  generates a signal indicative of an overlap and outputs it to the host PC  11  (Step S 79 ). Upon receiving this signal, the host PC  11  stores another value corresponding to the “overlap” such as “2” at the corresponding address MA K  within the overlap data memory address  183 . 
     FIG. 13 shows schematically this relationship between the paper position and the overlap data memory address. With reference to FIG. 13, if the overlap detection unit  12  outputs a signal indicative of a possibility of an overlap at paper position K=1, the value “1” is stored at MA 1  of the overlap data memory address  183 . If the overlap detection unit  12  outputs a signal indicative of an overlap at paper position K=2, the value “2” is stored at MA 2  of the overlap data memory address  183 . 
     In this example, a signal corresponding to value “1” is outputted at paper position K=2 before a second signal corresponding to value “2” is outputted. Accordingly, value “1” is initially set at MA 2  of the overlap data memory address  183  and thereafter when the signal corresponding to “2” is outputted, the value “1” already set at MA 2  is deleted and replaced by “2”. In other words, there is a correspondence between “K” of the paper position  181  and MA K  of the overlap data memory address  183 . These addresses MA K  are initially reset (in Step S 71 ) and contain 0. When it is determined that there is a possibility of an overlap at paper position K (or in the interval between positions K and K+ 1 ), value “1” is substituted at address MA K . If it is determined that there is an overlap in this interval, value “2” is substituted at address MA K . 
     If it is not an overlap phase difference (NO in Step S 74 ), if J is less than K2 (NO in Step S 78 ) or if the process of Step S 79  has been completed, the CPU  21  determines whether or not the paper sheet  41  being scanned has advanced by a specified distance (Step S 80 ). Explained more in detail, this is done by the CPU  21  calculating the distance of travel by the paper sheet  41  on the basis of the motor clock synchronization signals  182  supplied from the motor driver  24  through the processing unit  93 , or the cumulative number of rotations of the motor  25 . On the basis of this calculated distance, the CPU  21  determines whether or not the paper sheet  41  has advanced by a specified distance from the previous specified position K along its path of transportation or to the next specified position K+1. 
     Although FIG. 13 indicates a distance corresponding to two periods of the motor clock synchronization signal  182  as the specified distance, the length of this specified distance is not intended to limit the scope of the invention and may be arbitrarily set. 
     If it is determined that the paper sheet  41  has been advanced by the specified distance (YES in Step S 80 ), the paper position value K is incremented by 1, or K=K+1 (Step S 81 ). The incremented paper position value K is stored in the paper position area and a signal corresponding to this new paper position K is supplied to the host PC  11 . On the basis of this supplied signal corresponding to the paper position K, the host PC  11  (or its CPU  111 ) determines the address MA K  in the overlap data memory address  183  where value “1” or “2” should be substituted. In Step S 81 , the overlap frequency J of the judgment counter  97  is also reset to zero through the processing circuit  93  in order to check the possibility of an overlap in the next specified interval (from K+1 to K+2). 
     Next, if the CPU  21  determines that the paper sheet  41  is not being detected, or that its transportation has been completed (NO in Step S 82 ), the process is concluded. If the paper sheet  41  is still being transported and hence it is detected (YES in Step S 82 ), the CPU  21  returns to Step S 73  and repeats the subsequent Steps. 
     Thus, if it is determined in Step S 80  that the paper sheet  41  has not advanced by the specified distance and if it is further determined in Step S 82  that the paper sheet  41  has been detected, or, for example, if a possibility of an overlap is always present at each sampling time in the interval between paper positions K and K+1, the corresponding value “1” for a possibility or “2” for an overlap is substituted at MA K  in the overlap data memory address  183 . 
     In other words, it is not the possibility-indicating value at one paper position K that is substituted at address MA K  but the values corresponding to the interval between the paper positions K and K+1. As a result, as will be explained more in detail below, the host PC  11  can determine whether or not the entire surface of a paper sheet  41  is overlapped (from the front edge to the back edge) by referencing all of the addresses MA K  in the overlap data memory address  183 , and further determine therefrom whether or not an unwanted overlap has actually taken place. 
     FIG. 12 is a flowchart for another process which is a variation of the process explained above with reference to FIG.  11 . The flowchart of FIG. 12 is different from that of FIG. 11 only in that the overlap frequency J of the judging counter  97  is reset to zero through the processing unit  93  (Step S 103 ) when it is determined in Step S 74  that it is not an overlap. This program has the merit of more accurately judge the possibility of an overlap and an erroneous detection of an overlap can be prevented. 
     FIG. 14 is referenced next to explain an example of processing by the host PC  11  corresponding to the operations of the overlap detection unit  12  shown by FIG. 11 or  12 . 
     To start, the CPU  111  of the host PC  11  resets the page number P stored in the P-area (Step S 111 ) and sends out one of the paper sheets  41  on the path of transportation (or places it on the paper feeding plate  32 ) (Step S 112 ). Next, the CPU  111  increments the page number by 1, or P=P+1, and stores the incremented page number P in the P-area (Step S 113 ). 
     Next, the CPU  111  determines whether or not the paper sheet  41  has been brought onto the path of transportation, or whether or not the transportation of the paper sheet  41  has been started (Step S 114 ) and whether or not it has been transported out of the path, or whether or not its transportation has been completed (Step S 115 ). Details of Steps S 111 -S 115  are not repetitiously explained because Steps S 111 -S 115  are the same as Steps S 51 -S 55  of FIG.  7 . 
     If a possibility-indicating signal corresponding to value “1” is received from the overlap detection unit  12  as a result of Step S 77  in FIG. 11 or  12  before the transportation of the paper sheet  41  is completed (NO in Step S 115 ), however, the CPU  111  substitutes its value at the corresponding address MA K  in the overlap data memory address  183 . Similarly, if an overlap signal corresponding to value “2” is received from the overlap detection unit  12  as a result of Step S 79  in FIG. 11 or  12 , the CPU  111  substitutes this value at the corresponding address MA K  in the overlap data memory address  183 . 
     If it is determined in Step S 1115  that the overlap detection unit  12  is not detecting the paper sheet  41 , or that the transportation of the paper sheet  41  has been completed (YES in Step S 115 ), the CPU  111  reads out all of the values “1” and “2” as well as the initially set values “0” stored at the addresses MA K  of the overlap data memory address  183  (Step S 116 ). 
     Thereafter, the CPU  111  examines to determine whether value “2” is among the values read out in Step S 116  (Step S 117 ). If there is no “2” among the values read out (NO in Step S 117 ), it is further checked to determine whether or not value “1” is among the values that have been read out (Step S 118 ). If it is determined that there is “1” among the values that have been read out in Step S 116  (YES in Step S 118 ), the CPU  111  reads out the page number P stored in the P-area and stores it in the display area (Step S 119 ). 
     If no “1” is found to be among the values read out in Step S 116  (NO in Step S 118 ), or if the process of Step S 119  has been completed, the CPU  111  determines whether or not the transportation of all of the paper sheets  41  has been completed (Step S 120 ). If it is determined that it has not been completed yet (NO in Step S 120 ), the CPU  111  returns to Step S 112  and repeats the subsequent Steps, sending off the next sheet of paper  41  on the path of transportation and checking whether or not there is a possibility of an overlap. 
     If it is determined that all of the paper sheets  41  have been transported (YES in Step S 120 ), the CPU  111  causes data corresponding to the page numbers P determined to have a possibility of an overlap to be displayed on the output device (CRT)  117 . 
     There is no special limitation on the data to be thus displayed on the output device (CRT)  117  as long as the user can recognize therefrom the page number P determined to have a possibility of an overlap. For example, a display may be made as shown in FIG. 8,  9  or  10 . 
     If it is determined in Step S 117  that value “2” is present (YES in Step S 117 ), the CPU  111  examines whether or not MA K  are “1” or “2” over the entire surface of the paper sheet  41  (Step S 122 ). If “1” or “2” is substituted in all of MA K , or if each MA K  for the surface has “1” or “2” substituted therein (YES in Step S 122 ), the CPU  111  concludes that there is no “overlap” and proceeds to Step S 120 , repeating the subsequent Steps. 
     If not every one of MA K  is “1” or “2” on the surface and there is at least one MA K  for the surface having “0” substituted therein (NO in Step S 122 ), the CPU  111  concludes that there is an unintentional overlap and stops the transportation of the paper sheet  41  (Step S 123 ) and concludes the operation. 
     In summary, if a possibility of an overlap is found all over the surface of the paper sheet  41 , it is concluded that it is a doubly folded paper sheet or that two sheets of paper are intentionally overlapped and hence that it is not a situation where it should be concluded that there is an overlap in the ordinary sense. In other words, the overlap detector  1  according to this embodiment of the invention is capable of recognizing a doubly folded paper sheet and a pair of paper sheets intentionally overlapped one on top of the other. 
     Although the invention has been described by way of only a limited number of examples, these examples are not intended to limit the scope of the invention. For example, although the process shown by the flowcharts herein are intended to be carried out by software by the CPU  21  of the overlap detection unit  12  of FIG. 1 or the CPU  111  of the host PC  11 , it is naturally possible to prepare a hardware device for carrying out such processes. Although the invention was described above by way of examples applicable to a scanner, neither is it intended to limit the scope of the invention. The invention is equally applicable to the detection of overlapped paper sheets in a copier, a printer or apparatus of other kinds adapted to transport sheets of paper one by one. 
     The steps of writing programs to be stored in the memory medium include not only processes to be carried out in the order of writing but also processes which need not be carried out in the time sequence but may be carried out in parallel or independently. 
     In summary, the device and method of the present invention can inform the user whether or not an overlap in a true sense of the word is present.