Method and device for controlling banknotes

Abstract

 A method of controlling banknotes, the method including the steps of acquiring an image of a whole banknote; dividing (41) the acquired image into a number of predetermined regions, each having a number of pixels, and each pixel being assigned a respective shade value; adding (45) the shade values of pixels in each region to obtain a quantity related to the mean value of the shade values; comparing (56) the quantities related to the mean value with respective reference values; and generating a banknote pass/reject signal (S) on the basis of the outcome of comparing all the regions of the banknote.

Description

[0001] The present invention relates to a method and device for controlling banknotes.

[0002] As is known, banknotes are produced from special sheets (typically comprising watermarks and/or metal bands) large enough to accommodate several finished banknotes, and which are subjected to various printing steps, using different printing methods, to obtain the various graphic and alphanumeric characters. More specifically, printing may comprise some or all of the following steps:

a) Offset printing. This is performed out of line with the edge of the sheet, which therefore cannot be used as a reference by which to determine the coordinates of the offset-printed details. Offset printing is normally performed on both sides of the sheet.

b) Copperplate printing. This is performed at high pressure, may be displaced with respect to the offset printing, and slightly deforms the paper, thus possibly resulting in inclination of the copperplate with respect to the offset printing. Copperplate printing may only be performed on the front or both sides (front-back) of the banknote, and may comprise several steps, each of which may be horizontally/vertically misaligned or inclined with respect to the others and with respect to the offset printing.

c) Silk-screen printing. Like copperplate printing, this may be displaced or inclined with respect to the offset printing.

[0003] Following the above printing steps, the printed sheet is quality controlled, and only the passed banknotes are printed with serial numbers. Finally, the sheet is cut to separate the banknotes, but cutting is not performed in line with any of the printed details.

[0004] Quality control is currently performed manually to ensure the various printed details conform closely enough with an ideal value, and that there are no errors in colouring (too much ink or none at all), no smudges, etc.

[0005] At present, there is no way of automatically controlling the print quality of banknotes, in that, to take into account the numerous variables involved, the deviation thresholds used to compare the banknote with a specimen image would have to be so high that even banknotes with serious errors in colouring would be passed.

[0006] Automatic control systems do exist for validating, discriminating between, or determining the deterioration of banknotes already in circulation, but which provide for examining only a very small portion of the note (typically a narrow horizontal intermediate strip through significant parts of the overall design). The information supplied by such systems is therefore insufficient for quality control purposes, in which case the inking defects and smudges for detection are normally localized.

[0007] It is an object of the present invention to provide a reliable, automatic method and device for controlling banknotes.

[0008] According to the present invention, there is provided a method of controlling banknotes, characterized by comprising the steps of acquiring an image of a whole banknote, and comparing said acquired image with a reference image of a whole specimen banknote.

[0009] According to the present invention, there is also provided a device for controlling banknotes, characterized by comprising acquisition means for acquiring an image of a whole banknote; and comparing means for comparing said acquired image with a whole reference image of a specimen banknote.

[0010] A non-limiting embodiment of the present invention will be described by way of example with reference to the accompanying drawings, in which:

Figure 1 shows an overall block diagram of a device in accordance with the present invention;

Figure 2 shows a flow chart of the method according to the present invention;

Figures 3 and 4 show block diagrams of details in Figure 1;

Figure 5 shows a plan view of a specimen banknote indicating specific lines used in the method according to the present invention.

[0011] Number 1 in Figure 1 indicates as a whole a device for quality controlling banknotes 2 printed on a sheet 3.

[0012] More specifically, control device 1 comprises a television camera 4 for picking up one banknote at a time, and for generating and supplying a digitized discrete grey-tone television signal to an image memory 5. Image memory 5 memorizes the image of banknote 2 in the form of a matrix of dots (pixels), each of which is assigned a value (also referred to hereinafter as a shade value) related to the grey level (luminance) of the pixel.

[0013] Image memory 5 is connected to an image processor 6 for performing a first processing operation of the image of banknote 2, and for determining the coordinates of the image with respect to a predetermined reference system used later for comparison with a specimen banknote. Image processor 6 is therefore connected to a specimen memory 7, by which it is supplied with selected portions of a specimen banknote, which are compared with similar portions of the image to be repositioned. The output of image processor 6 is connected to an edge extractor 8, which receives the shifted image of the banknote to be controlled, and processes the shifted image to generate a processed image, the pixels of which define the edges of the drawings and alphanumeric characters on the banknote, and the portions of the banknote having a brightness gradient with respect to the adjacent portions.

[0014] Edge extractor 8 is connected to a local averaging and comparing unit 9 for locally averaging the processed image received from edge extractor 8, and for making a local comparison with corresponding image portions of the specimen banknote - also averaged - supplied by specimen memory 7. Local averaging and comparing unit 9 also processes the local comparison results of all the portions, and supplies, at an output 10, a signal S accepting or rejecting the controlled banknote 2. A control unit 11 is connected to units 5-9 to control the operation sequence as well as any processing parameters.

[0015] With reference to Figure 2, the control method shown comprises a first step of acquiring and memorizing the image of a whole banknote by means of camera 4 and image memory 5 (block 12). From the acquired image (block 13), image processor 6 selects a number of small predetermined regions containing predetermined significant characteristics of the banknote, taking into account any position inaccuracy resulting from displacement of the banknote with respect to the theoretical position, and from the printing deviations described previously. For example, the predetermined regions may be such as to definitely contain the edge portions defined by lines A and B in Figure 5.

[0016] The selected predetermined regions of the camera image are processed to extract the significant characteristics (lines A and B) of the banknote (block 14), for example, as described in detail later on with reference to Figure 3 showing edge extraction by edge extractor 8.

[0017] Image processor 6 then redefines the position of the banknote with respect to the reference system of device 1 (used for the specimen banknote) using the position of the extracted significant characteristics and the reference position of the same significant characteristics on the specimen banknote (block 15). For example, image processor 6 may determine, in known manner, the horizontal and vertical deviation of the extracted predetermined significant characteristics with respect to the same significant characteristics on the specimen banknote, and calculate correct coordinates of the banknote on the basis of the deviation, or may use known rotation-translation algorithms.

[0018] The repositioned image of the banknote is then supplied to edge extractor 8, which processes the image by filtering it through an edge detection convolution filter (block 16), e.g. a 3x3 kernel filter as shown in Figure 3 and described in detail later on.

[0019] The processed image, by now only containing the edges and pixels with brightness gradients with respect to the adjacent regions, is then sent by extractor 8 to local averaging and comparing unit 9, which processes the image to add the values of pixels in predetermined regions. More specifically, unit 9 divides the banknote into a number of predetermined portions, and adds the shade values of the pixels in each portion to obtain a number of values, one for each portion and each proportional to the mean shade value of that portion (block 17). These values are then compared with corresponding shade values of the specimen banknote (processed beforehand in the same way as for the banknote being controlled) to determine the deviation (block 18); and the detected deviations as a whole are processed according to predetermined criteria governing acceptance or rejection of the banknote (block 19). For example, the banknote may be passed if all the detected deviations are below a predetermined threshold, or if a significant portion (e.g. 90%) of the deviations is below a first threshold, and the rest are anyway below a second higher threshold.

[0020] If the banknote is passed (YES output of block 19), a pass signal is generated (block 20); conversely (NO output), a reject signal is generated (block 21); which signals may be used for printing the serial numbers (which, as stated, are only printed on the passed banknotes) and for separating the passed banknotes from the rejects when sheet 3 is cut.

[0021] Figure 3 shows a diagram of the convolution filter for extracting the edges in block 14 of Figure 2. More specifically, the filter, which is substantially known and indicated as a whole by 22, comprises two FIFO registers 23, 24; a multiplication matrix 25 with nine cells 26-34 arranged in three rows and three columns; four adders 35-38; an input line 39; and an output line 40. Input line 39 is connected to the input of cell 26 and to the input of register 23; the output of register 23 is connected to the input of register 24 and to the input of cell 29; the output of register 24 is connected to the input of cell 32; each cell 26, 29, 32 has two outputs, a first connected to the input of adder 35, and a second connected to the cell on the right (27, 30, 33 respectively); each cell 27, 30, 33 has two outputs, a first connected to the input of adder 36, and a second connected to the cell on the right (28, 31, 34 respectively); each cell 28, 31, 34 has an output connected to the input of adder 37; and the outputs of adders 35, 36, 37 are connected to the inputs of adder 38, the output of which is connected to output line 40.

[0022] Cells 26-34 of filter 22 provide for multiplying the input pixel value by a predetermined value (8 for cell 30 and -1 for cells 26-29 and 31-34) and for supplying it to the respective adders; and cells 26-31 supply the value of the same pixel (unchanged) to the next cell in the same row. For each clock count, therefore, a new-pixel value is supplied to cell 26 and register 23; the "oldest" pixel in register 23 is supplied to cell 29 and register 24; the "oldest" pixel in register 24 is supplied to cell 32; cells 26, 29, 32 supply adder 35 with the result of multiplying the pixel received in the previous clock count, and supply the same pixel received previously (unchanged) to respective next cells 27, 30, 33; similarly, cells 27, 30, 33 supply the multiplication result to adder 36, and the unchanged pixel value to next cells 28, 31, 34; cells 28, 31, 34 simply supply the multiplication result to adder 37; adders 35, 36, 37 supply the sum of the previous values to adder 38; and adder 38 supplies the total value to the output.

[0023] In the example shown, assuming the banknotes are scanned in columns, and that each register 23, 24 memorizes a number of pixels equal to that of one column (e.g. 128), cells 26-34 receive, at each clock count, the values of a central pixel and the eight surrounding pixels, and multiply them by the coefficients indicated; and the multiplication results are then added so that each pixel in the image is modified according to the value of the eight adjacent pixels. Consequently, in the case of uniform portions (same pixel values), each pixel is assigned a zero or, at any rate, a low value, whereas the pixels in edge or high-contrast portions are assigned high values, thus transforming the original image into a processed image containing practically only edges, and in which the value assigned to the pixels belonging to the edges indicates the degree of contrast or gradient with the adjacent pixels.

[0024] An example embodiment of local averaging and comparing unit 9 will now be described with reference to Figure 4.

[0025] Unit 9 comprises a selecting element 41 having an input 42 serially supplying the pixel values processed by image processor 6, and a number of outputs 43, each connected to a respective local section 44. Each local section 44 comprises an adder 45 having a first input 46 connected to a respective output 43 of selecting element 41, a second input 47, and an output 48 connected to an accumulator 49 having two control inputs 50, 51 respectively receiving an enabling signal EN and a reset signal RES. Accumulator 49 also has an output 52 connected to input 47 of adder 45 and to a memorizing element or latch 53 having an enabling input 54 receiving a respective control signal L, and an output 55 connected to a first input of a comparator 56, which also has a second input connected to a reference buffer 57 for memorizing a local reference value and which is enabled by a control signal B. Comparator 56 also has an input receiving a control signal C, and an output 58 connected, like all the outputs of local sections 44, to a logic unit 59, which, depending on the outcome of the comparisons made in all the local sections 44, determines acceptance or rejection of banknote 2 in block 19 of Figure 2. In the example shown, control signals EN, RES, L, B and C are supplied by control unit 11 in Figure 1.

[0026] In local averaging and comparing unit 9, the pixels - scanned, for example, in columns - are supplied to selecting element 41, which distributes them, in predetermined groups, to sections 44. For example, if local averaging and comparison are performed in 8x8 pixel regions, selecting element 41 sends the first 8 pixels in the first column to the first section 44, the next 8 pixels in the first column to the second section 44, and so on up to the end of the column, and then sends the first 8 pixels of the second column to first section 44, the second 8 pixels in the second column to second section 44, and so on. In each section 44, the value of each pixel received at input 46 is added by adder 45 to the previous total supplied at input 47, and the sum is memorized in accumulator 49 enabled, at this step, by signal EN. The sum is repeated for all the received pixels of eight successive columns, and the total is memorized in latch 53 enabled for the purpose by signal L; accumulator 49 is reset by signal RES to memorize the sum of the next region from zero; and reference buffer 57 supplies a reference value REF (corresponding to the sum of the specimen banknote pixel values in the same region, the image of which has been processed in the same way as described above to extract the edges) to comparator 56, which, as stated, supplies the local comparison value defining a local-error signal E for use by logic unit 59.

[0027] Selecting element 41 may operate in different ways to add the pixel values (local averaging) depending on the control precision required and the characteristics of the banknotes being controlled. For example, as opposed to 8x8 pixel regions as described above, local averaging and comparison may be performed in 16x16 pixel regions. Alternatively, the banknote may be divided into predetermined areas, even differing in size, so as to contain whole copperplate details; in which case, the banknote may be repositioned according to displacement of the offset printing, and the areas for comparison must be large enough for each to definitely contain the respective copperplate detail, taking into account any displacement of the copperplate with respect to the offset printing. Alternatively, repositioning may be performed according to displacement of the copperplate printing, and the areas for comparison may be the same size as the copperplate details, and therefore smaller than previously.

[0028] In another solution, the local averaging and comparison regions may be of predetermined size (e.g. 8x8) in the portions containing only offset or copperplate printing, and of larger size in the combined regions.

[0029] Finally, it should be pointed out that, as opposed to being performed prior to extraction, the banknote repositioning step may be performed after the edge extracting step, using already extracted edge portions; and convolution to extract the edges may be performed using a software or hardware filter.

Claims

1. A method of controlling banknotes, characterized by comprising the steps of acquiring an image of a whole banknote, and comparing said acquired image completely with a reference image of a whole specimen banknote.

2. A method as claimed in Claim 1, characterized by comprising the steps of:

- dividing said acquired image into a number of predetermined regions, each comprising a number of pixels; each said pixel being assigned a respective shade value;

- calculating a quantity related to the mean value of the shade values of the pixels in each said region; and

- comparing said quantities related to said mean values with respective reference values.

3. A method as claimed in Claim 2, characterized in that said step of calculating a quantity related to the mean value comprises the step of adding said shade values of the pixels in said region.

4. A method as claimed in Claim 2 or 3, characterized in that said predetermined regions are of predetermined constant size.

5. A method as claimed in Claim 2 or 3, characterized in that said banknote comprises print details; and in that said predetermined regions are of a size related to said print details.

6. A method as claimed in any one of the foregoing Claims from 2 to 5, characterized in that said dividing step is preceded by a step of extracting edges of representations in said image.

7. A method as claimed in Claim 6, characterized in that said step of extracting edges comprises the step of filtering said image by means of a convolution filter.

8. A method as claimed in any one of the foregoing Claims, characterized in that said comparing step is preceded by a step of repositioning said banknote with respect to a predetermined reference system.

9. A method as claimed in Claim 8, characterized in that said repositioning step comprises the steps of acquiring the shade values of pixels in predetermined portions of said image; determining predetermined significant characteristics in said predetermined portions; and determining shifted coordinates of said image on the basis of the deviation between said predetermined significant characteristics and corresponding reference significant characteristics of said specimen banknote.

10. A device for controlling banknotes, characterized by comprising acquisition means (4, 5) for acquiring an image of a whole banknote (2); and comparing means (9) for fully comparing said acquired image with a whole reference image of a specimen banknote.

11. A device as claimed in Claim 10, characterized by comprising selective acquisition means (41, 44, 46) for selectively acquiring pixel shade values in predetermined regions of said image; calculating means (45) for calculating a quantity related to the mean value of the pixel shade values acquired in each of said predetermined regions; local comparing means (56) for locally comparing each said quantity related to said mean value with a respective reference value, and for generating a number of local-error signals (E); and a logic unit (59) connected to said local comparing means, and for generating a pass/reject signal (S) on the basis of said local-error signals (E).

12. A device as claimed in Claim 11, characterized in that said selective acquisition means (41, 44, 46) comprise a selecting element (41) and a number of calculating sections (44); said selecting element (41) comprising an input (42) sequentially receiving said pixel shade values, and a number of outputs (43) selectively supplying groups of said pixel shade values; each said calculating section (44) comprising an adding element (45) having a first input (46) connected to a respective output (43) of said selecting element (41), a second input (47), and an output (48) connected to said second input (47); a memorizing element (53) connected to said output (48) of said adding element (45) and having an output (55); and a respective local comparator (56) having a first input connected to said output (55) of said memorizing element (53), a second input receiving said reference value (REF), and an output (58) connected to said logic unit (59).

13. A device as claimed in any one of the foregoing Claims from 10 to 12, characterized by comprising edge extracting means (8) between said acquisition means (4, 5) and said comparing means (9).

14. A device as claimed in Claim 13, characterized in that said edge extracting means (8) comprise a convolution filter (22).

15. A device as claimed in any one of the foregoing Claims from 10 to 14, characterized by comprising repositioning means (6) between said acquisition means (4, 5) and said comparing means (9); said repositioning means (6) comprising selecting means (13) for selecting predetermined portions of said image; extracting means (14) for extracting predetermined significant characteristics (A, B) of said image in said predetermined portions; and means (15) for determining correct coordinates of said banknote on the basis of a comparison between said predetermined significant characteristics of said image and corresponding predetermined significant characteristics of said reference image.

Drawing