Abstract:
A method of monitoring a sequence of documents passing along a transport path is described. The method including: operating a radiation transmitter with a control signal at a first, working, level to cause radiation at a first intensity to impinge on one side of a document as it passes an inspection position in the transport path; receiving at a radiation receiver, radiation from the transmitter that has passed through the document, the radiation receiver generating an output signal with a level related to the intensity of the received radiation; and monitoring the output signal to detect the presence and/or a characteristic of the document. A calibration process is carried out between successive documents, the calibration process including: operating the radiation transmitter with a control signal at a second, calibration, level to cause radiation at a second intensity less than the first intensity to be transmitted towards the receiver, and adjusting the level of the resultant output signal from the receiver to a predetermined value.

Description:
FIELD OF THE INVENTION 
     The invention relates to a method of monitoring a sequence of documents passing along a transport path. The invention is particularly concerned with the monitoring of security documents such as banknotes, cheques and the like but could also be used for monitoring other types of document. 
     PRIOR ART 
     In a conventional document monitoring system, one inspection technique which is commonly used is to cause a beam of radiation to pass through each document so as to monitor the attenuation of the radiation beam. This attenuation is related to the thickness of the document and can be used to detect the presence of overlapped or double fed documents which is usually undesirable. Once such an overlapped condition has been detected then the documents can be routed to a cull pocket or the like. In some cases, however, particularly where the documents are to be counted, even the overlapped documents could be validly counted. A example of this type of device is described in WO-A-00/42477. 
     An important aspect of this type of monitoring device is that it must be calibrated in order to ensure that a preset detection level at the receiver monitoring when the transmitted radiation is sufficiently attenuated to correspond to the passage of one or more documents, is not affected by soiling etc. Conventionally, calibration is achieved by placing a dampening film between the transmitter and the receiver while the transmitter is operated at full power. The received power level is noted and if necessary an adjustment can be made to either the full power supplied to the transmitter or to the gain used at the receiver. The dampening film is then removed and documents can be processed. 
     An important variable which it has been found needs to be taken into account, particularly with ultrasonic transmitters, is the effect of temperature changes. These can cause the intensity of the emitted radiation to change for a given control signal input. U.S. Pat. No. 4,406,996 describes an optical detection device which attempts to avoid the problems of temperature changes as well as the presence of dust and the like. In this system, before any documents are fed, the transmitter is activated at full power level and then the power level is gradually decreased until the received intensity reaches a preset threshold. The system is then ready for operation. 
     We have found, however, that in the high speed handling of documents such as banknotes, typically at speeds of 6-8 notes per second, the temperature within the device can vary significantly even during the processing of a single batch of 100 documents. The process of U.S. Pat. No. 4,406,996 is not able to deal with this rapid temperature variation. 
     EP-A-0921083 describes another example of an optical detection device. Initially, when no note is present between the lamp and sensor, the lamp generates illumination at an initial brightness well below its maximum level. The brightness is then adjusted until the output from the sensor is maximised. The drawback of this approach is that it is necessary to control the lamp intensity as part of the calibration process. 
     WO-A-92/17857 describes yet another optical detector in which groups of LEDs are provided on one side of a document path while strip photodiodes are provided on the other side. The LEDs are energised sequentially. The output level from each LED is the same during both calibration and note detection and during calibration the gain of an amplifier connected to the LEDs is adjusted so that its output is constant in the absence of a document. 
     GB-A-2165045 discloses a further optical detector in which a light source is energised at a constant level and the output from the sensor, fed through an amplifier, is monitored and the gain of the amplifier adjusted occasionally if during the calibration phase it falls below a reference level. 
     One of the difficulties with these approaches is that the sensor needs to be sensitive to relatively low radiation levels when a document is present but during calibration, when no document is present, the sensor is exposed to the full power of the source of radiation. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a method of monitoring a sequence of documents passing along a transport path comprises: 
     operating a radiation transmitter with a control signal at a first, working, level to cause radiation at a first intensity to impinge on one side of a document as it passes an inspection position in the transport path; 
     receiving, at a radiation receiver, radiation from the transmitter that has passed through the document, the radiation receiver generating an output signal with a level related to the intensity of the received radiation; and 
     monitoring the output signal to detect the presence and/or a characteristic of the document; 
     and is characterised by carrying out a calibration process during the passage of documents along the transport path and when a gap between documents exists at the inspection position, the calibration process comprising 
     operating the radiation transmitter with a control signal at a second, calibration, level to cause radiation at a second intensity less than the first intensity to be transmitted towards the receiver, and adjusting the level of the resultant output signal from the receiver to a predetermined value. 
     With this invention, we overcome the problems mentioned above by operating the radiation transmitter at two very different levels. This enables the output level from the receiver, typically controlled by controlling the gain of an amplifier, to be adjusted during the calibration phase in response to a relatively low level of radiation (the calibration level) instead of utilizing the higher intensity of radiation (the working level) where any amplifier would typically saturate. Thus, the invention provides a simple two level control to the radiation transmitter and deals with fine adjustment of the receiver by suitably adjusting the output signal from the receiver. 
     In a typical example, the resultant output signal during calibration or when a document is present and thus absorbing a substantial proportion of the working level intensity radiation will be in the order of 100 microvolts. Where the receiver includes an amplifier, typical amplifiers will saturate at 400 microvolts. Thus, during calibration, a typical calibration level control signal will have a voltage of 200 mV peak to peak whereas the corresponding working level will be 20V. In general, the magnitude of the first, working level will be at least 10 times the magnitude of the second, calibration level. 
     More generally, the second, calibration level of the control signal will be up to about 500 times less than the first, working level of the control signal. This simulates the level of the signal received by the receiver in the presence of a document. 
     In addition, in this new method, we calibrate the transmitter/receiver pair during the passage of the documents. This may be between the passage of each document or group of documents. This ensures that even rapid temperature variations are compensated. Furthermore, the particular way of achieving that calibration enables the process to be carried out between documents or groups of documents even though they may be fed at high speed. That is, the calibration process requires the simple switching of a control signal between its working and calibration levels and this should be contrasted with the process described in U.S. Pat. No. 4,406,996 which requires a sequential stepping of the output power of the transmitter from its full power level down to the preset level. This would take too long to carry out between successively fed documents at the feed rates contemplated. Typically, the calibration process will take 200 μs-1 ms while the time between successive banknotes of conventional dimensions fed at 6-8 notes per second, long edge leading is of the order of 50 ms. 
     Although the invention is applicable to a single radiation transmitter/receiver pair, preferably a plurality of transmitter/receiver pairs are provided, wherein the calibration process comprises controlling all the transmitters with the control signals at the same calibration level, and adjusting the signals output by the receivers to the same predetermined value. 
     The plurality of transmitter/receiver pairs will typically be provided in a line transverse, typically orthogonal, to the direction of the transport path. This enables characteristics of the documents to be detected at different positions across the transport path. 
     In addition to carrying out the calibration process between successive documents, the process may also be carried out before documents begin to be fed along the transport path, so as to achieve an initial calibration. 
     The control and output signals will typically comprise a voltage or a current level depending upon the way in which the transmitter is operated and the receiver configured. 
     The invention is primarily concerned with radiation in the ultrasound frequency range typically in the range 40 kHz-1 MHz, particularly 300 kHz, but is also applicable to radiation in other wavelength bands, particularly optical, infrared and ultraviolet. 
     Other information can be obtained from the radiation transmitter/receiver pair. For example, a gap between successive documents can be detected when the receiver detects radiation at the first intensity. Alternatively, gaps between successive documents can be detected using other methods such as an optical detector or simply by predicting when a gap is expected knowing the speed at which documents are transported along the transport path. 
     The presence of a document can be detected when a control signal at the second, calibration level is applied to the transmitter and the corresponding receiver receives substantially no radiation. 
     In some cases, it is advantageous if the or each transmitter is controlled with a control signal set at a level less than the first, working level, such as the calibration level, as long as the radiation from that transmitter does not impinge on a document. In the event of a document arriving, the intensity of the ultrasound received at the receivers will decrease substantially to zero and the first scan line of the documents surface will be lost, but thereafter the transmitter will be operated by a control signal at the working level. As the height (i.e. length in the transport direction) of a scan line is small compared to the height of a document, the loss of data for a single line will be negligible. 
     The method can be used to detect the thickness of each document as it passes the inspection position as well as folded, or overlapped documents. In addition, or alternatively, other characteristics of the documents could be detected such as the presence of tapes, closed and open tears and the like. 
     The invention is particularly applicable to the processing of documents of value such as banknotes but could be used for monitoring the passage of other documents as will be readily apparent to a person of ordinary skill in the art. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An example of a document inspection apparatus for carrying out a method according to the present invention will now be described with reference to the accompanying drawings, in which: 
         FIGS. 1A and 1B  are schematic plan and side elevations of a document inspection station forming part of the document handling apparatus; 
         FIG. 2  is a flow diagram illustrating operation of the document thickness monitoring components; 
         FIG. 3  is a flow diagram illustrating the calibration process of  FIG. 2 ; 
         FIG. 4  is a flow diagram illustrating the processing of intensity signals during the passage of a document through the inspection station; 
         FIG. 5  illustrates a signal path model for a transmit and receive ultrasonic transducer pair; 
         FIG. 6  illustrates schematically the signals obtained before and after compensation during the temperature compensation calibration process; 
         FIG. 7  illustrates the signals obtained before and after compensation during note scanning; 
         FIG. 8  illustrates the variation in sensitivity with temperature for a typical commercially available transducer; 
         FIG. 9  illustrates the way in which the output signals from each of 16 ultrasonic transducers in an array varies depending upon the ambient temperature; and, 
         FIG. 10  illustrates the variation in the output signal generated by the adjustable gain amplifiers after they have been calibrated. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     The document handling apparatus shown in  FIGS. 1A and 1B  will be described with reference to a banknote receiving device although the invention is applicable to other types of banknote handling equipment such as banknote sorters and recyclers. The apparatus includes an inspection station  10  ( FIGS. 1A and 1B ) through which banknotes  12  are fed as a result of being conveyed by a pair of conveyor belts  14 . 
     The inspection station  10  comprises an array of 16 ultrasonic transmitting transducers  16  located below the transport path and a corresponding array of 16 ultrasonic receiving transducers  18  located above the transport path and positioned such that each ultrasonic receiver is located directly opposite a corresponding ultrasonic transmitter. The two arrays  16 ,  18  extend orthogonally to the direction of movement of the banknotes as indicated by an arrow  20 . It would also be possible for the arrays  16 , 18  to extend in a non-orthogonal direction across the transport path. 
     Suitable transducers are manufactured by Murata under the product name MA300D1. 
     Each ultrasonic transmitting transducer  16  is controlled by control signals fed along respective control lines  22  from front end electronics  24 . The front end electronics  24  are in turn controlled from a suitably programmed computer  26 . 
     Each ultrasonic receiving transducer in the array  18  is connected via a respective output line  28  to signal conditioning electronics  30  including an adjustable gain amplifier (not shown in  FIG. 1A ), the output of which is digitized and then fed along a respective output line  32  to the computer  26 . The computer  26  generates gain adjustment signals in the manner to be described below, these gain adjustment signals being fed along respective lines  34  to the conditioning electronics  30 . Other processing arrangements are possible to implement these functions as will be readily understood by a person of ordinary skill in the art. For example, the adjustable gain amplifier could be a distinct component or form part of the computer  26  in the form of a digital signal processor. 
     Hereafter, a transmitting transducer and associated electronics will be referred to as a transmitter while a receiving transducer and associated electronics will be referred to as a receiver. 
     The computer  26  also outputs signals on a line  36  relating to the characteristics of banknotes  12  which have been monitored in the inspection station. 
     The process undertaken by the components at the inspection station will now be described with reference to  FIGS. 1   a ,  1   b  and  2 . Initially, before a batch of banknotes  12  is transported by the transport system through the inspection station, a calibration operation will be carried out (step  40 ,  FIG. 2 ) to be described in more detail below. Once the calibration process has been carried out, the transport system will be activated and the first document  12  will be fed by the conveyor belts  14  to and through the inspection station  10  (step  42 ). 
     Once activated, the transmitters  16  are repeatedly activated in groups of odd and even numbered transmitters (to reduce the risk of cross-talk) at the low, calibration level (to be described in more detail below). This is achieved by providing appropriate voltage control signals at the “calibration” level to the appropriate transmitters. Alternatively, the transmitters could be controlled by controlling the current supply but account must be taken of the fact that output intensity does not vary directly with current amplitude. 
     When a document arrives at the inspection station  10 , the intensity of ultrasound received at the receivers  18  will decrease substantially to zero allowing the computer  26  to determine that the document has arrived (step  44 ). 
     The computer  26  then adjusts the control signals to the transmitter  16  to their high “working” level and again sequentially pulses the transmitters in their odd and even groups. The document will then be monitored (step  46 ) as described in more detail below. 
     During the passage of a document  12  between the transmitters and receivers  16 ,  18 , the intensity of the transmitted ultrasonic signal will be attenuated but when the ultrasound is transmitted through a gap between documents then the received intensity will increase substantially. The computer  26  detects this increase in intensity to determine the presence of a gap (step  48 ). 
     While a gap between documents is present, the computer  26  controls the components to carry out a further calibration operation (step  50 ). 
     The computer  26  then determines whether the banknote  12  just monitored is the last banknote in the batch. This may be determined if it is known in advance how many banknotes are to be transported or, for example, if the time between completion of monitoring a banknote and the time at which the next banknote is expected is exceeded (step  52 ). 
     If the document just monitored is not the last document then processing returns to step  44  but otherwise the transport is stopped (step  53 ). 
     When a document  12  is being monitored (step  46 ) as it passes through the inspection station  10 , the output signals from the ultrasonic receivers in the array  18  are regularly sampled (step  100 ,  FIG. 4 ) so as to build up (step  102 ) a map of the apparent opacity to ultrasonic radiation of different pixels of the banknote and this can then be used to determine characteristics about the banknotes such as the presence of a tape, tears, folds and overlapped banknotes and the like (step  104 ), for example by determining that certain groups of pixels attenuate the ultrasound by more than a certain threshold. Depending upon the result of that comparison, the computer  26  will issue a suitable output signal on the line  36  which can be used to control further processing of the note in a conventional manner. 
     Briefly, the calibration process carried out in steps  40  and  50  is set out in  FIG. 3 . During this calibration process, the voltage control signal transmitted along the lines  22  is set to pulse the transmitters successively in odd and even groups at a calibration level (step  54 ) which is lower, usually by about 100 times (e.g. 200 mV), than the voltage (e.g. 20V) of the control signal used when monitoring banknotes such that the intensity of the resultant ultrasound signal transmitted by each transmitter of the array  16  is about 100 times less than its working level. In other cases, the calibration level could be up to 500 times less than the working level. 
     The output signal then generated by each ultrasonic receiver is compared by the computer  26  with a common predetermined value (step  55 ) and the computer then calculates an adjustment that needs to be made to the gain of the adjustable gain amplifier corresponding to each ultrasonic receiver so as to modify the resultant output signal fed to the computer  26  to take up the predetermined value (step  56 ). 
     As a result of this calibration process, it is anticipated that for a given thickness of banknote, the output signal generated from the adjustable gain amplifier corresponding to each ultrasonic receiver will be substantially the same thus allowing a simple comparison with a predetermined common threshold corresponding to the expected signal from an intact, single banknote. 
     The signal generation and processing will now be described in more detail.  FIG. 5  provides a simplified model of one ultrasonic transmit and receive transducer pair. As can be seen, the transmit ultrasonic transducer  60  receives a control signal having a voltage U in  on the line  22  via a transmit amplifier  62 . The resultant ultrasonic signal is transmitted across the note path and is received by a receive ultrasonic transducer  64  which generates a corresponding output signal with a voltage which varies with sensed ultrasonic intensity and which is amplified by a receive amplifier  66  and then integrated by integrator  68  before being fed to an adjustable gain amplifier  70 . The components  66 ,  68 ,  70  are located in the processing electronics  30 . The effect of the adjustable gain amplifier  70  is to adjust the level of the signal output from the integrator to a calibrated value. 
     As explained earlier, the system in fact has  16  independent channels, which all possess a signal path according to the above description. The gain factors of the corresponding adjustable amplifiers  70  can be set independently while all other amplifiers will be assigned a type-specific mutual constant (i.e. g tx  and g rx ). 
     This model is based on the assumption that multiplying g tx  by a factor will multiply the received signal by the same factor. It reflects the temperature-dependency of the elements in or adjacent the acoustic section of the signal path by including a temperature parameter for the affected functions. A thorough description of all the temperature dependencies will be very complex but we believe the model set out above and shown in  FIG. 5  is sufficient for the purpose of understanding and implementing this example of the invention. 
     As explained above, the calibration process involves three steps. 
     1. When the air gap between the transmitter  60  and the sensor  64  is unobstructed (e.g. before the batch processing starts or in the note gap), the output power level of the transmitter array is reduced (c*g tx ) by generating control signals at the low calibration level  61  ( FIG. 6 ) (e.g. 200 mV), and the measured signal intensity on the receiver side stored for each channel. The resulting signal intensity on the receiver side is in this case approximately the same as is measured in working level mode with a banknote in the air gap (same operation point). 
     2. A gain correction factor for each channel (i) is computed according to the formula:
 
gainFactor( i )=TargetLevelConstant/storedIntensityForChannel( i )
 
 FIG. 6  illustrates schematically the process at this stage. The graphs  72  illustrate the output U out  from each integrator  68  and it can be seen that these outputs vary relative to a mean output level  74 . The adjustable gain amplifiers  70  are therefore adjusted by the computer  26  so that the final output from each adjustable gain amplifier  70  is at the same value  74  as shown by graphs  76 .
 
     In a modified approach, instead of computing the gain values solely based on data for the current note, the processing history could be taken into account. One possible implementation of this could be a moving average. 
     Before the transport starts, a background scan at calibration level is performed. The measured channel intensity mean values are used as seed for the moving average. In each note gap, the channel intensities are measured at calibration power level. The gains for correcting the image are then computed by updating the moving average with the just-measured note-gap intensities. This is repeated for all notes in the bundle being processed. 
     3. As the banknote arrives at the inspection station  10  (during note scanning), the transmitter array&#39;s output is reset to the working level (e.g. 20V) as described above. For each measurement point of the note surface (pixel of the ultrasonic image) the intensity is multiplied with the gain factor of the relevant channel:
 
outputIntensity( i )=measuredIntensity( i )*gainFactor( i )
 
       FIG. 7  illustrates the system and signals during note scanning. As can be seen, the control signals  63  applied to each transmitter in the array  16  are at the higher, working voltage level. In this case, the banknote  12  carries a length of tape  80  which will cause increased attenuation of the transmitted ultrasound. The effect of this can be seen in the graphs  82  where the received intensities from the outermost receivers is at a very high level because the path from the corresponding transmitters is unobstructed. For those receivers which receive ultrasound having been transmitted through both the banknote  12  and the tape  80 , the received signal level is at a very low level about a mean  81  while the remaining receivers receive ultrasound radiation which has passed only through the banknote  12  generating outputs about a mean intermediate level  83 . These output levels are then adjusted by the amplifiers  70  so that the output signal levels corresponding to each respective condition are the same as shown by graphs  84 . 
     As explained above, the transport of documents such as banknotes at high speed causes significant temperature variation within the apparatus and the transmit and receive transducers in the arrays  16 , 18  are susceptible to variation due to the temperature changes.  FIG. 8  illustrates the variation in sensitivity with temperature for a typical commercially available transducer while  FIG. 9  illustrates the way in which the output signals from each of 16 ultrasonic receivers in an array corresponding to the array  18  varies depending upon the ambient temperature. These signals correspond to those output from each integrator  68 . In this case, the same control signal was applied to each transmitting transducer at all temperatures. 
     The problem with this variation is that the variation (or noise) exceeds variations expected during the passage of banknotes. 
       FIG. 10  illustrates the variation in the output signal generated by the adjustable gain amplifiers  70  after they have been calibrated for the same 16 receivers and it will be seen that the variation is much smaller than for the uncompensated signals.