Abstract:
An optical detector is disclosed, which is adapted to measure the opacity of media. The detector comprises a light means and a light sensor, arranged so as to have a media path there between. The light source has a drive means, which is actively adjustable, during use, for detecting media of different opacities, so as to maintain a substantially constant sensor output.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     The present invention relates generally to an optical sensor and a method of operation thereof and in particular to a method of enhancing sensor accuracy.  
         [0002]     Optical sensors are commonly used for a variety of functions including detecting skewed or double picked notes within the note transport mechanism of an Automated Teller Machine.  
         [0003]     A variety of different prior art detectors have been utilized to detect note skew in ATMs. These include both electromechanical and optical detectors. However, they all have certain features in common. In particular, they all rely on a pair of sensors, each of which is located at a predetermined position along the transport path within the ATM. Also as the detector is arranged to determine skew perpendicular to the direction of travel along the transport path, both sensors and light sources must be located within the transport path, thus making assembly and serviceability of the detectors difficult. For example, cables must be laid into both sides of the transport path to connect to the sensors.  
         [0004]     In addition, changes in LED power and sensor sensitivity throughout the lifetime of a sensor have also caused problems when attempting to use optical sensors for note detection in an ATM.  
         [0005]     A further problem with the use of optical sensors is the large variation in the opacity of notes used today. Also, some bank notes have relatively transparent windows. With prior art optical sensors these windows are seen as holes.  
       SUMMARY OF THE INVENTION  
       [0006]     It is an object of the present invention to provide an optical sensor that ameliorates the aforementioned problems.  
         [0007]     It is a further object of the present invention to produce an improved note skew detector.  
         [0008]     It is a still further object of the present invention to provide an optical sensor that can operate accurately while utilizing a relatively inexpensive phototransistor-as opposed to an expensive photo-diode.  
         [0009]     According to a first aspect of the present invention there is provided an optical detector adapted to measure the opacity of media, comprising a light means and a light sensor, arranged so as to have a media path there between, the light source having a drive means which is actively adjustable, during use, for detecting media of different opacities, so as to maintain a substantially constant sensor output.  
         [0010]     Preferably, the optical sensor is a single optical sensor.  
         [0011]     Most preferably, the light source and optical sensor are optically coupled via two distinct optical paths, which are formed in part by optical light guides.  
         [0012]     Preferably the detector comprises a control means arranged to make determinations as to the degree of skew of a note based on the signal produced from the sensor.  
         [0013]     Preferably, when in use, the detector is arranged such that the sensor receives light via each optical path, the output of the sensor being dependent on whether or not a note is present in either or both optical paths.  
         [0014]     According to a second aspect of the present invention there is provided an Automated Teller Machine (ATM) having an optical detector as described above.  
         [0015]     According to a third aspect of the present invention there is provided a method of detecting the opacity of media utilizing a detector comprising a sensor, a light source and associated drive means arranged to provide a media path therebetween, the method comprising 
    a) passing media therebetween,     b) adjusting the current to the light source in order to maintain the output of the sensor at a substantially constant level, and     c) measure the current required as a measure of opacity of the media being detected.    
 
         [0019]     According to a fourth aspect of the present invention there is provided a method of detecting skew in a bank note, being transported along the transport path of a note transport mechanism, utilizing an optical detector comprising a light source and an optical sensor, which are optically coupled via light guides arranged to transmit light from the source to the sensor via two distinct optical paths, comprising detecting the actively adjustable input to the light source, required during use, for media of different opacities, so as to maintain a substantially constant sensor output an output at the sensor corresponding to both the first and second optical paths.  
         [0020]     According to a fifth aspect of the present invention there is provided a method of detecting double picked bank notes in an ATM transport mechanism, utilizing an optical detector comprising a light source and an optical sensor, which are optically coupled via light guides arranged to transmit light from the source to the sensor via two distinct optical paths, comprising detecting the actively adjustable input to the light source, required during use, for media of different opacities, so as to maintain a substantially constant sensor output an output at the sensor corresponding to both the first and second optical paths. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]     An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
         [0022]      FIG. 1 ; is a schematic illustration of a note skew or double pick detector in accordance with the present invention;  
         [0023]      FIG. 2  is a schematic illustration of the detector of  FIG. 1  in the transport mechanism of an Automated Teller Machine (ATM) in accordance with the present invention;  
         [0024]      FIG. 3  is a graphical representation of the variable output of a prior art detector, during the detection of a bank note;  
         [0025]      FIG. 4  is a graphical representation of the detector output produced to maintain a substantially constant sensor output when zero, one or more media pass through the detector;  
         [0026]      FIG. 5  is a schematic representation of the drive circuitry of a sensor in accordance with the present invention; and  
         [0027]      FIG. 6   a  is an illustration of the output of a sensor in accordance with the present invention when a single note is detected; and  
         [0028]      FIG. 6   b  is an illustration of the output of a sensor in accordance with the present invention when two notes are detected. 
     
    
     DETAILED DESCRIPTION  
       [0029]      FIG. 1  illustrates a skew note detector  10 , including an optical sensing means  12 , for use in a note transport mechanism  14  of an Automated teller Machine (ATM) (not shown). The detector  10  comprises a light source  16  and a single optical sensor  18 , optically coupled via a pair of optical wave-guides  20 A,  20 B. The wave-guides are arranged to have an air gap  22  there between, so as to provide a note transport path between the said wave-guides. The wave-guides are further arranged to provide a first optical path  24 A and a second, distinct, optical path  24 B between the light source  16  and the sensor  18 . In this way the output of the sensor  18  is dependent on the light transmitted via the wave-guides  20 A,  20 B to the detector  18 , over both optical paths  24 A,  24 B. The output of the sensor  18  is fed to a control means  25  arranged to make determinations as to the degree of skew of a note based on the output of the sensor  18 , as will be discussed in more detail below, with reference to  FIGS. 2 &amp; 3 .  
         [0030]      FIG. 2  illustrates the use of the detector  10  in the transport mechanism  14 . In addition it illustrates the flexibility of the detector which, in addition to note skew detection can also provide information on double picked notes. The cash transport mechanism of  FIG. 2  is part of an ATM cash dispensing mechanism, comprising a currency cassette  26  arranged to contain a stack of currency notes  28  of the same pre-determined denomination supported on their long edges. The cassette  26  is associated with a pick mechanism  30 . When one or more currency notes are to be dispensed from the cassette  26  in the course of a cash dispensing operation, the pick mechanism  30  draws out notes one by one from the stack  28 , and each note is fed by feed rollers  32 , 34 , 36  via guide means  38  to feed rollers  40 . The direction of feed of the notes is at right angles to their long dimensions. It should be understood that the cash dispensing mechanism  14  could include more than one cassette each associated with a pick mechanism, but in the present embodiment only one cassette and pick mechanism will be described.  
         [0031]     Each picked note is passed through the sensing station  12  by the feed rollers  40  and by further feed rollers  42 . If a multiple note is detected by the optical system  10 , in a manner to be described in more detail below, then a divert gate  44  diverts the multiple note via rollers  46  into a reject bin  48 , in a manner known to a skilled person.  
         [0032]     If a single note is detected then the note passes on to a stacking wheel  50  to be loaded on to stationary belt means  56 . The stacking wheel  50  comprises a plurality of stacking plates  52  spaced apart in parallel relationship along the shaft  51  of the stacking wheel  50 . When the required number of notes have been loaded on to the belt means  56 , the belt means  56  transports the notes to a cash delivery slot (not shown), again in a manner known to a skilled person, which will not therefore be described further herein.  
         [0033]     The detector  10  is positioned within the transport mechanism  14 , such that the first and second wave-guides  20 A,  20 B lie on opposite sides of the transport path. Thus one or more bank notes being transported by the mechanism will pass through the air gap  22  between the wave-guides  20 A,  20 B. As the source  16  and sensor  18  are arranged at the same side of the transport path all necessary wiring can be located at the one side making assembly and repair considerably easier than in prior art detectors. Hence there is no need to feed wiring into the body of the transport mechanism, as with prior art skew and double pick detectors.  
         [0034]      FIG. 3  illustrates the output of a prior art non-compensated detector. To obtain maximum contrast between zero, one and two notes the light is set and fixed to an intensity that gives maximum sensor output with no notes present i.e. close to ground or supply. When a note is introduced the light reaching the sensor is reduced, generally from 100% to 5%. The output is now close to the signal noise level. By introducing a further note a similar (20 times) reduction will take place. Output is now 0.25% and cannot be easily discriminated from noise. Thus it can only be said that there is more than one note. Such a system will fail with more opaque media such as Thin Film media.  
         [0035]     Also, changes in operation of the light sources or sensors used in such detectors during their lifetime can cause comparable changes in output from detectors leading to false readings.  
         [0036]      FIG. 4  illustrates a detector output in accordance with the present invention in which the output of the sensor is maintained at a constant level by adjusting the supply voltage of the light source when one or more notes is detected.  
         [0037]     When no notes are present the output of the detector is maintained at a fixed, low level, say 300 mV by applying a current of 0.12 mA to the light source within the detector. In order to maintain the same sensor output, when a note is placed in the optical path between the light source and the sensor, the current supplied to the light source must be raised, say to 8.0 mA. If a second, superposed note is located between the light source and sensor the input must be raised again, to say 30 mA, in order to maintain the same output from the sensor.  
         [0038]     Thus the change in input from zero to one note is almost a 7-fold increase and the increase from one to two notes is more than 4-fold. Thus these increases are much more easily determined than with prior art methods. Thus measuring the input to the light source instead of the output from the sensor provides an improved detector.  
         [0039]     With more powerful light sources these current levels would be greater and more linear, therefore, allowing the detection of extremely opaque media.  
         [0040]      FIG. 5  illustrates the feedback circuit required to enable the maintenance of a constant sensor output, in the detector in accordance with the present invention.  
         [heading-0041]     The Compensated Opacity Schematics  
         [heading-0042]     The Loop Reaction Speed Depends On:  
         [0043]     The charge current delivered from the driver circuit to the charge capacitor The efficiency of the LED. Higher efficiency demands less current and thus speeds up the charge of the charge capacitor as well as it demands less change in a given situation and thus speeds up the loop reaction.  
         [0044]     The phototransistor load resistor. A smaller load resistor (greater load) depletes the base region of the phototransistor faster and allows for a faster turn off.  
         [0045]     The load of the charge capacitor. The smaller the two resistors R 3  and R 4  are the faster the charge capacitor can be depleted.  
         [0046]     The charge capacitor. A smaller capacitor is charged and depleted faster.  
         [0047]     The inductor. A larger inductor increases the drive current.  
         [heading-0048]     Closed Loop  
         [0049]     The LED (D 4 ) and the phototransistor (U 2 ) are physically positioned such that U 2  receives light from D 4 . This light path, together with the FB input of U 1 , creates a closed loop. The loop balances when the voltage U FB  to GND is approximately 0.252 [V].  
         [heading-0050]     Reduction of Light  
         [0051]     By reducing the photo current in U 2  (reduction of light received by U 2 ) the voltage U FB  is reduced. This result in a current increase delivered by U 1  and thus (over time) a voltage increase across C 1  which in turn results in a current increase in D 2 , D 3 , D 4 , R 4  and R 3 . A current increase in D 4  (white LED) gives a rise in the light produced and equilibrium is restored. As this results in a current increase in R 3  the output voltage increases with the light increase.  
         [0052]     Over Voltage Protection and Maximum Current  
         [0053]     U 1  has a built-in over voltage protection circuit, which prevents the voltage across C 1  from rising beyond 27.5 [V].  
         [0054]     The maximum current that can pass through D 4  is thus given by 
 
 I   D4max =( U   OVP   −U   D2+D3+D4 )/( R   3   +R   4 )=(27.5−(0.7+0.7+4))/(68+270)=65 [mA]
 
 Maximum Output Voltage 
 
         [0056]     The maximum output voltage is given by the maximum current through R 3 . 
 
 U   o     —     max   =I   D4max *R 3 =0.065*68=4.42 [V]
 
 Avoiding closed loop oscillations 
 
         [0058]     If U 1  is capable of charging C 1  faster than U 2  can change the photo current then the feed back voltage (U FB ) will change too slowly and a U C1  overshoot will be the result which in turn gives excess D 4  current and thus excess light.  
         [0059]     The rise time created by U 2  and its load resistor (R 2 ) must be so much smaller than the charging of C 1  that the resultant overshoot can be accepted. The actual speed with which C 1  is charged by U 1  depends on a set of factors which depends on the efficiency of the boost converter formed by U 1 /L 1 . Experiments are needed to obtain these data. A good result is achieved for R 2 =100 k, L 1 =5.6 uH and C 1 =10 uF.  
         [heading-0060]     LED On Time  
         [0061]     When a more opaque media is introduced into the light path the feed-back loop increases the LED current to compensate for the measured light loss. The LED ON time depends on the speed with which the driver can increase the drive voltage (charge the charge capacitor) and thus the LED current. This in turn depends on the maximum drive current and the size of the charge capacitor.  
         [0062]     A larger capacitor reduces the ON time at the delivered current and vice versa.  
         [0063]     The current being delivered depends on the inductor. A larger inductor increases the current. The driver is limited to handle inductors below 27 uH.  
         [0064]     By using over current (70 mA versus 20 mA) the LED On Time is reduced. The total light path must be so efficient that a common bill results in a LED current of 20 [mA] or less. The light path should not permanently be obstructed as this will lead to decreased lifetime.  
         [0065]     The higher the LED efficiency is the less current is used to create light and similarly more current is available to charge the charge capacitor.  
         [heading-0066]     LED Off Time  
         [0067]     The speed with which the light output will be reduced depends on the capacity of C 1  given that U 1  can switch off in a few microseconds.  
         [0068]     The C 1  discharge path depends on R 3  and R 4  assuming that the forward voltages of the diodes are reasonably constant. 
 
τ= R*C= (68 +270)*10 u=3.38 [ms]
 
         [0069]     This is too slow. A τ of less than 0.3 [ms] is wanted.  
         [0070]     This can be achieved by increasing max current. A higher max current will result in smaller resistors. However a higher max current stresses the LED! This also demands a faster phototransistor/resistor pair as C 1  will charge faster.  
         [0071]     Modifications may be incorporated without departing from the scope of the present invention.