Abstract:
An apparatus for detection of multiple documents ( 10 ) in a document transport system comprises a transmitter ( 15 ), located on a first side of a document feed path ( 40 ), and emits a signal through the document feed path. A reflector ( 52 ), located on a second side of said document feed path, reflects the signal through the document feed path. A first sensor detects the signal from the transmitter. A second sensor detects the signal from the transmitter.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    Reference is made to commonly-assigned copending U.S. patent application Ser. No. 10/303,263, filed Nov. 25, 2002, entitled A METHOD AND APPARATUS FOR DETECTION OF MULTIPLE DOCUMENTS IN A DOCUMENT SCANNER USING MULTIPLE ULTRASONIC SENSORS, by Phinney et al., the disclosure of which is incorporated herein. 
     
    
     
       FIELD OF THE INVENTION  
         [0002]    This invention relates in general to using multiple detectors for sensing multiple document feeds or small documents overlapped with larger documents.  
         BACKGROUND OF THE INVENTION  
         [0003]    Scanners and copiers use document feeders to transport documents into the machine. Mechanisms used for the transportation of documents, including paper or sheets of other material, have the capacity to accidentally pick up more than one document fed from a stack of documents. It is necessary to determine when more than one document is pulled into a document transport since multiple documents may jam the transport or prevent processing some documents. In many cases, the documents fed into the scanner are different sizes. Some smaller documents may be located on one side of a document feed path and not pass under a single position sensor.  
           [0004]    There are two general methods for multiple document detection, contact and non-contact. The contact methods include measurement of small thickness changes with a contact foot or sensing arm that is in contact with the documents as they pass through the document transport. The contact foot is connected to a linear voltage differential transducer (LVDT), or a magnet, which is sensed by a Hall Effect Sensor. These sensors can detect changes in thickness of less than 1 μm (10 −6  m).  
           [0005]    The major disadvantage to the contact method is that anything in contact with moving paper, especially thin paper or ripped paper, can cause a malfunction such as a paper jam. The contact method also requires calibration using the maximum thickness document that will be fed through the document transport. When a thickness is measured which is above the calibration value plus a threshold, typically 30%, it is determined to be a multiple document feed. This method, however, will only work when documents having a uniform thickness are processed. Using a wheel on the end of the contact foot can reduce the chances of paper jam, however, the variations in the diameter of this wheel, due to the nonconformity in manufacturing, must be taken into account during the measurements.  
           [0006]    The primary non-contact method for multiple document detection sends ultrasound signals through the document stream to determine if more than one document is present. Sending ultrasound through paper results in attenuation of the ultrasound signal. It is possible to determine the presence of multiple documents by change in attenuation of the signal received. It is also possible to detect multiple documents in a document feed path by changes in the phase of the signal transmitted through the documents. This method is independent of the thickness of the individual documents and is made without making contact with these documents.  
           [0007]    There are currently ultrasonic detection system available, which use high frequency sensors to sense multiple zones within a local area. This approach works because the sensors are directional, and the signal from one sensor does not interfere with the signal from other sensors. However, these sensors are also more expensive. Low cost sensors have a wide angle of energy emitted, and if used, the sensors can interfere with each other (cross-talk). The interference often causes the design to fail. The problem cannot be solved by multiplexing individual pulses to the emitters, because not enough energy will be sensed by the detector and the time phasing of the signal is not stable.  
           [0008]    U.S. Pat. No. 6,212,130 uses a tilted ultrasonic sensor. This could be used in multiple locations but the cost would be significant, using the more expensive directional high frequency sensors. Other approaches using various combinations of transmitters and sensors add to the cost of the document transport system since both transmitters and sensor are expensive. It is desirable to provide a low-cost yet accurate multiple document detection system using as few components as possible and inexpensive components.  
         SUMMARY OF THE INVENTION  
         [0009]    Briefly, according to one aspect of the present invention, an apparatus for detection of multiple documents in a document transport system comprises a transmitter, located on a first side of a document feed path, which emits a signal through the document feed path. A reflector, located on a second side of the document feed path, reflects the signal back through the document feed path. A first sensor detects the reflected signal from the transmitter. A second sensor, detects the reflected signal from the transmitter. The first and second sensors are located on the same side of the document feed path as the transmitter.  
           [0010]    In one embodiment of the present invention the transmitter also comprises a detector which detects the reflected signal and provides an additional point of detection of multiple documents. In another embodiment of the present invention a transmitter transmits a signal at a wide angle through the document feed path which is detected by multiple sensors at various positions along a width of the document feed path. A reflector, located on a side of the feed path opposite the transmitter, runs substantially the entire length the document feed path.  
           [0011]    The invention and its objects and advantages will become more apparent in the detailed description of the preferred embodiment presented below. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0012]    [0012]FIG. 1 is a schematic diagram showing paper passing through the system with a double passing through zone A.  
         [0013]    [0013]FIG. 2 s a schematic view showing multiple reflectors.  
         [0014]    [0014]FIG. 3 is a schematic view showing a transmitter located at one edge of a document feed path.  
         [0015]    [0015]FIG. 4 is a schematic diagram showing a wide angle transmitter with the wall opposite the transmitter comprising a reflective surface. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    The present invention will be directed in particular to elements forming part of, or in cooperation more directly with the apparatus in accordance with the present invention. It is to be understood that elements not specifically shown or described may take various forms well known to those skilled in the art.  
         [0017]    Referring now to FIG. 1, an apparatus for multiple document detection  10  in accordance with one embodiment of the present invention is shown. In this particular embodiment, the apparatus includes an ultrasonic drive circuit  11 , which provides a drive signal to transmitter  15 , as determined by the microprocessor  21 . The ultrasonic transmitter produces an ultrasonic signal  30  that passes through a document feed area  40 . One or more documents  42  may be transported through document feed area  40 . The transmitted ultrasonic signal  30  is reflected by reflector  52 . The reflected ultrasonic signal again passes through document feed path  40 , and any documents  42  which are in the feed path. The reflected signal is received by ultrasonic receivers  16 ,  18 .  
         [0018]    A phase shift of the reflected ultrasonic signal is relatively independent of the thickness of the document or documents in the document feed. This results in a received ultrasonic signal with a phase shift approximately dependent on only the number of documents in the document feed, because of the interfaces between different materials through which the ultrasound passes causes the phase shift, not the total thickness of the documents. While detection of a phase change in the reflected ultrasonic signal is the preferred method of multiple document detection, amplitude detection changes may also be used to detect multiple documents. A combination of amplitude detection and phase change may also be incorporated to add reliability to the multiple document detection system.  
         [0019]    The ultrasonic receivers  14 ,  18  convert the reflected ultrasonic signal into an electrical signal. The electronic signal is sent to a phase comparator  20  wherein the phase difference between the drive signal and the electronic signal is determined. An information signal  50  which represents the determined phase difference is fed from phase comparator  20  to a microprocessor  21 .  
         [0020]    The microprocessor  21  checks the appropriate information signal to determine if multiple documents are present based on the resulting phase shift or difference between the drive signal and the electronic signal. The microprocessor  21  may also be programmed to detect an amplitude change in the received signal. The change in amplitude of the received signal may also be used to determine the presence of multiple documents. The microprocessor may also be programmed to determine if there are multiple documents present based either on the amplitude signal, the phase change, or a combination of one or both of the signals.  
         [0021]    The microprocessor  21  checks for double document feed between receivers  14 ,  18 . The presence of a multiple document detected at receiver  14  or receiver  18  indicates multiple documents in feed path  40 . The presence of multiple documents may be used to shut down the document feed transport or sound an alarm to summon an operator.  
         [0022]    Although a microprocessor  21  is shown, other types of processors or programmable devices can also be used. Additionally, although in this particular example, an ultrasonic signal is used in this apparatus, other types of signals can also be used.  
         [0023]    Referring now to FIG. 2, an alternate embodiment of the invention is shown wherein multiple reflectors  54 ,  55 , and  56  are located on a side of the document feed path  40  opposite transmitter  15 . Transmitter  15  transmits a wide angle signal which is reflected off reflectors  54 ,  55 ,  56 . The reflectors are placed at a location and at an angle such that the signal  30  is reflected to an appropriate receiver  14 ,  18 . In this embodiment, as for other embodiments of this invention, transmitter  15  may also receive reflected signals. Used in this fashion the transmitter/receiver  15  provides an additional location in the document feed path  40  for detection of multiple documents.  
         [0024]    [0024]FIG. 3 shows an alternate embodiment of the invention wherein transmitter  15  is located at one edge of document feed path  40 . In this embodiment, receivers  14 ,  18  and transmitter  15  are all equally spaced across document feed path  40 . The reflectors are spaced and angled in an appropriate fashion to provide optimal reflection path from the transmitter to the receivers.  
         [0025]    [0025]FIG. 4 is yet another embodiment of the present invention in which the side of the document feed path  40  opposite transmitter  15  is comprised of a reflective material. Transmitter  15  provides a wide angle signal which is reflected off reflector  57  through multiple paths to the receivers. This embodiment eliminates the requirement for separate reflectors, which may further reduce the cost and complexity of the document transport system.  
         [0026]    The invention has been described in detail with particular reference to certain preferred embodiments thereof, but it will be understood that variations and modifications can be effected within the scope of the invention.  
       Parts List  
       [0027]    [0027] 10 . Multiple document detection  
         [0028]    [0028] 11 . Ultrasonic drive circuit  
         [0029]    [0029] 14 . Receiver  
         [0030]    [0030] 15 . Transmitter/Receiver  
         [0031]    [0031] 18 . Receiver  
         [0032]    [0032] 20 . Phase comparator  
         [0033]    [0033] 21 . Microprocessor  
         [0034]    [0034] 30 . Ultrasonic signal  
         [0035]    [0035] 40 . Document feed path  
         [0036]    [0036] 42 . Documents  
         [0037]    [0037] 50 . Information signal  
         [0038]    [0038] 52 . Reflector  
         [0039]    [0039] 54 . Reflector  
         [0040]    [0040] 55 . Reflector  
         [0041]    [0041] 56 . Reflector