Patent Publication Number: US-2004057051-A1

Title: Surface reflectivity discriminating device

Description:
FIELD OF THE INVENTION  
       [0001] The present invention relates to a discriminating device for discriminating between two kinds of objects based on surface reflectivity differences thereof, and more specifically to a device for discriminating between printing-plate and interleaf paper.  
       BACKGROUND OF THE INVENTION  
       [0002] In CTP devices for direct imaging on printing plates, the plates are accommodated in a magazine, or cassette and provided one by one to be exposed by the imaging device.  
       [0003] The plates provided in the cassette are usually separated by interleaf paper, interposed between the plates, to prevent friction damages to the plates&#39; emulsion-covered surfaces.  
       [0004] In the course of imaging plates, the plate placed at the top of the stack is picked and transferred to the exposure area for imaging. If an interleaf paper (slip-sheet) is placed at the top of the stack, the paper is picked and disposed of.  
       [0005] Accordingly, discriminating means for discriminating between plate and paper are used, to correctly identify the topmost object on the stack.  
       [0006] Published application EP 1136403 describes a discriminating device comprising two optical sensors. One of the sensors reacts to light reflected by both plate and paper, while the second sensor reacts only to light reflected by a plate. The first and the second sensors are placed on the optical axis of the light reflected from the sensed surface. Both sensors react to a predetermined intensity of reflected light. The difference is that the second sensor is inclined and its predetermined intensity is lower, in order to react to polymer surface only.  
       [0007] The disadvantage of the method of EP 1136403 lies in its low discrimination factor (‘paper’ to ‘metal’ signal ratio), which would require intensity (threshold) adjustments per batch of plates of the same type having different reflectivity, on top of intensity adjustment per plate-type.  
       [0008] There is need for a discriminating device that overcomes the shortcomings of existing devices. The required device should be independent of variations in material and of sub-variations within plates of the same material.  
       SUMMARY OF THE INVENTION  
       [0009] The proposed surface discriminating method and device are free of the drawbacks of existing solutions, due to extended ratio of plate sensing to paper sensing signals, covering all differences between different materials and batches of the same material.  
       [0010] According to one aspect of the present invention, there is provided an apparatus for discriminating between objects having different surface reflectivity, comprising:  
       [0011] a light source for illuminating an object;  
       [0012] a first optical sensor mounted relative to said light source, said first optical sensor receiving direct light reflected from said object;  
       [0013] a second optical sensor mounted relative to said light source, said second optical sensor receiving dispersed light reflected from said object;  
       [0014] means for subtracting a light signal received by said second optical sensor from a light signal received by said first optical sensor, said means for subtracting connected with said first and second optical sensors; and  
       [0015] means for determining said object type, said means for determining connected with said means for subtracting.  
       [0016] The apparatus may additionally comprise a first amplifier connected with said first optical sensor and a second amplifier connected with said second optical sensor, wherein said first amplifier and said second amplifier are connected with said means for subtracting.  
       [0017] The means for determining may comprise a comparator and a reference, wherein said comparator compares an input thereof, received from said subtracting means, with said reference.  
       [0018] According to another aspect of the present invention, there is provided a CTP device for imaging printing plates stacked in a cassette, said plates separated by paper sheets, comprising:  
       [0019] an imaging system;  
       [0020] a first picking device for picking a topmost plate from said cassette and transporting said plate to said imaging system;  
       [0021] a second picking device for picking a topmost paper sheet from said cassette and transporting it to a disposal means; and  
       [0022] discriminating means for determining the type of a topmost object in said cassette, said discriminating means comprising:  
       [0023] a light source for illuminating said topmost object;  
       [0024] a first optical sensor mounted relative to said light source, said first optical sensor receiving direct light reflected from said topmost object;  
       [0025] a second optical sensor mounted relative to said light source, said second optical sensor receiving dispersed light reflected from said topmost object;  
       [0026] means for subtracting a light signal received by said second optical sensor from a light signal received by said first optical sensor, said means for subtracting connected with said first and second optical sensors; and  
       [0027] means for determining said topmost object type, said means for determining connected with said means for subtracting.  
       [0028] The apparatus may additionally comprise a first amplifier connected with said first optical sensor and a second amplifier connected with said second optical sensor, wherein said first amplifier and said second amplifier are connected with said means for subtracting.  
       [0029] The means for determining may comprise a comparator and a reference, wherein said comparator compares an input thereof, received from said subtracting means, with said reference.  
       [0030] The comparator&#39;s output is a logic high when said topmost object is a plate and a logic low when said topmost object is a paper sheet, or vice versa.  
       [0031] The first picking device and the second picking device may comprise a single picking device.  
       [0032] The first sensor and the second sensor may be mounted on a horizontal plane or on a sloped plane.  
       [0033] According to yet another aspect of the present invention, there is provided a method of discriminating between objects having different surface reflectivity, comprising the steps of:  
       [0034] providing a light source for illuminating an object;  
       [0035] providing a first optical sensor mounted relative to said light source, said first optical sensor receiving direct light reflected from said object;  
       [0036] providing a second optical sensor mounted relative to said light source, said second optical sensor receiving dispersed light reflected from said object;  
       [0037] subtracting a light signal received by said second optical sensor from a light signal received by said first optical sensor; and  
       [0038] determining said object type based on said step of subtracting.  
       [0039] According to yet another aspect of the present invention there is provided a method of determining the type of a topmost object in a plate loading cassette of a CTP device, comprising the steps of:  
       [0040] providing a light source for illuminating said topmost object;  
       [0041] providing a first optical sensor mounted relative to said light source, said first optical sensor receiving direct light reflected from said topmost object;  
       [0042] providing a second optical sensor mounted relative to said light source, said second optical sensor receiving dispersed light reflected from said topmost object;  
       [0043] subtracting a light signal received by said second optical sensor from a light signal received by said first optical sensor; and  
       [0044] determining said topmost object type based on said step of subtracting.  
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0045] For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, purely by way of example, to the accompanying drawings.  
     [0046] With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. In the accompanying drawings:  
     [0047]FIG. 1 is a schematic representation of a discriminating device according to a first embodiment of the present invention;  
     [0048]FIG. 2 is a functional scheme of a discriminating device according to the present invention; and  
     [0049]FIG. 3 is a schematic representation of a discriminating device according to a second embodiment of the present invention.  
    
    
     DETAILED DESCRIPTION OF THE PRESENT INVENTION  
     [0050]FIG. 1 is a schematic representation of a discriminating device according to the present invention, consisting of light source  100 , for example light emitting diode (LED) such as SFH484, available from Siemens of Munich, Germany, two optical sensors  120  and  130 , such as SFH-235, also available from Siemens of Munich, Germany and an object  110  to be detected, having a reflective surface.  
     [0051]FIG. 2 is a more detailed functional scheme of a discriminating device according to the present invention. Light source  100  is energized by current source  140 . A first sensor  120  is mounted so as to receive the direct reflection of the LED beam. Sensor  120  is connected through a first amplifier  150  to the input  1  of subtracter  170 . The second sensor  130  is mounted outside of the optical axis of the reflective light, at some distance from sensor  120 , preferably 2-4 cm, in order to detect the dispersed portion of the reflected light. Sensor  130  is connected through a second amplifier  160  to the input  2  of subtracter  170 . The output of subtracter  170  is connected to the first input of comparator  180 . The second input of the comparator  180  is connected to reference  190 . In another preferred embodiment, an additional amplifier may be placed between subtracter  170  and comparator  180 , to adjust the subtracter  170  output to the comparator  180  input. In yet another preferred embodiment of the present invention, subtracter  170  may be substituted with a divider or a multiplier (not shown).  
     [0052] Referring back to FIG. 1, the distance between the sensors  120  and  130  is determined based on the following considerations: Sensor  120 , intended to detect the directly reflected portion of the light, should be placed on the optical path of the directly reflected beam BC. The shorter the optical path ABC, the stronger will be the signal to noise ratio, which is desirable. Sensor  130 , intended to detect the dispersed portion of the light, should be placed outside the area AEFGH of direct light reflection. It can be mounted, for example, on the other side of the light source  100 , as shown in FIGS. 1 and 3, or on the same side, as shown in FIG. 2. The tradeoff in the placement considerations of sensor  130  is between decreasing the optical distance DB, from the sensor  130  to the direct beam reflecting point, which helps to increase the signal to noise ratio, and keeping the sensor  130  far enough from the light source so as not to fall within the direct light reflection area AEFGH and to be far enough from the investigated surface so that a big enough dispersed portion of light reaches it.  
     [0053]FIG. 3 is a schematic representation of another embodiment of the discriminating device of the present invention. The only difference between the embodiments of FIG. 1 and FIG. 3 is in the layout of the light source  100  and the sensors  120  and  130 . In the embodiment of FIG. 3 those elements are laid out on a sloped surface, so that the vertical distance of sensor  120  from the detected surface is smaller comparing to that shown in FIG. 1, so the sensor signal will be stronger. On the other hand, the vertical distance of sensor  130  from the detected surface is big enough to receive enough dispersed light.  
     [0054] The device operates as follows:  
     [0055] After power up, constant current from the current source  140  causes the lighting of LED  100 . This light falls onto detected surface  110 . If the detected surface  110  is smooth, like a plate, then the directly reflected portion Ds of the light will be big and the dispersed portion Ss will be small. If the detected surface is rough, like paper, then the directly reflected portion Dr of the light will be smaller than Ds and the dispersed portion Sr will be bigger than Ss.  
     [0056] The first and second sensors&#39; signals are amplified by first amplifier  150  and second amplifier  160 , respectively.  
     [0057] The output signal of the second amplifier  160  is subtracted from the output signal of the first amplifier  150  in subtracter  170 . When a plate is detected, the result of the subtraction (DsSs) yields a high value (because, as mentioned above, a low signal is subtracted from a high signal). When paper is detected, the result of the subtraction (Dr-Sr) yields a low value (because, as mentioned above, a high signal is subtracted from low signal).  
     [0058] In other words, when detecting plate, the result of the subtraction yields a high level signal and, when detecting paper, the result of the subtraction yields a low level signal. Comparator  180  compares the subtraction result with medium level reference  190 . If the detected surface is a plate, then the comparison will result with a logic high and if the detected surface is paper, then the comparison will result with a logic low, or vice versa if reverse logic is used.  
     [0059] In a preferred embodiment of the present invention, the amplifiers  150  and  160  have different amplification coefficients, chosen so as to maximize the difference between subtracter  170  outputs for plate and paper, thus increasing the discrimination factor.  
     [0060] Let Vsns1 and Vsns2 be the outputs voltage of direct ( 120 ) and dispersed ( 130 ) light sensors respectively.  
     [0061] Let A1 and A2 be the amplification factors of first ( 150 ) and second ( 160 ) amplifiers respectively.  
     [0062] Let U1 and U2 be the output voltage of first ( 150 ) and second ( 160 ) amplifiers respectively, where:  
     [0063] U1=A1*Vsns1  
     [0064] U2=A2*Vsns2  
     [0065] When a plate is sensed:  
     [0066] U1(pl)=A1*Vsns1(pl)  
     [0067] U2(pl)=A2*Vsns2(pl)1  
     [0068] The output of subtracter  170  will be:  
     [0069] IND(pl)=U1(pl)−U2(pl)  
     [0070] When paper is sensed:  
     [0071] U1(pa)=A1*Vsns1(pa)  
     [0072] U2(pa)A2*Vsns2(pa)  
     [0073] The output of subtracter  170  will be:  
     [0074] IND(pa)=U1(pa)−U2(pa)  
     [0075] A1 and A2 will preferably be chosen so as to maximize the ratio IND(pl):IND(pa).  
     [0076] It will be appreciated that although the embodiments of the present invention were described in conjunction with a plate loading system for CTP, the present invention lends itself to any discriminating device for discriminating between two kinds of objects based on surface reflectivity differences thereof.  
     [0077] It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined by the appended claims and includes both combinations and sub-combinations of the various features described hereinabove as well as variations and modifications thereof, which would occur to persons skilled in the art upon reading the foregoing description.