Overlapping detecting method for sheets of uneven density

Abstract: A sheet overlapping detecting method for use with sheets of unstable density includes the following steps. Light is radiated through a sheet of paper by N light emitting means and detected by N light detecting means, respectively, which generate a corresponding signal. The maximum light intensity signal is then selected and compared with each remaining N-1 signals to generate corresponding difference values associated with each N-1 signal respectively. The difference values are then compared with a predetermined value d and the corresponding light intensity signals of all difference values which exceed this value are rejected. The average value Dr of the remaining light intensity values is then calculated and compared with a prestored overlapping determination value to determing whether overlapping of sheets has occurred.

Description: Background of the Invention The present invention relates to a sheet overlapping detecting method for use in particularly a sheet-fed press. Conventionally, when sheets (sheets of paper) are to be fed to a sheet-fed press, overlapping of the sheets of paper to be fed is detected. That is, when a sheet of paper is to be fed from a feeding apparatus to a sheet-fed press (to be referred to as simply a press hereinafter), the leading edge of the sheet of paper is brought into contact with a stopper called a front guide provided at the press-side distal end portion of a feeding table, and then the sheet of paper is fed to the press. In this case, in order to prevent two or more overlapped sheets of paper from being simultaneously fed, a light-emitting device is arranged at the rear surface side of the feeding table in a position close to the front guide, and a through hole is formed in a predetermined portion of the feeding table corresponding to a light-emitting portion of the light-emitting device. In addition, a light-receiving device is arranged at the upper surface side of the feeding table corresponding to the through hole. That is, light emitted from the light-emitting device is radiated in the direction of thickness of a sheet of paper to be fed, and transmission light transmitted through the sheet of paper is received by the light-receiving device and converted into an electrical signal to obtain a received light amount. An output level corresponding to the received light amount is compared with a predetermined determination level, and overlapping of sheets of paper is detected on the basis of the comparison result. In this case, a light emission amount of light emitted from the light-emitting device and the determination level are preferably set to be optimum values for a corresponding sheet of paper. That is, the relationship between the light emission amount and the output level obtained when the number of sheets or paper is one is different from that obtained when the the number of sheets of paper is two. For example, as shown in Fig. 1, a characteristic curve I is obtained for one sheet of paper, and a characteristic curve II is obtained for two sheets of paper. In this case, the optimum value of the light emission amount is a light emission amount value at which a difference between output levels based on the characteristic curves I and II becomes maximum. The optimum value of the determination level is 1/2 a sum of the output levels based on the characteristic curves I and II obtained at the optimum light emission amount value. According to a first conventional method, a predetermined determination level is set, and power supply to the light-emitting device is adjusted such that the determination level is positioned at a substantially intermediate point between an output level obtained via the light-receiving device when the number of sheets of paper is one and an output level obtained when the number of sheets of paper is two, thereby setting a light emission amount of light to be emitted from the light-emitting device. According to a second conventional method, predetermined power supply to the light-emitting device is set to determine a light emission amount of light to be emitted from the light-emitting device, and a determination level is set to be positioned at a substantially intermediate point between an output level obtained via the light-receiving device when the number of sheets of paper is one and an output level obtained when the number of sheets of paper is two. In general, however, a small number of lots of a material is often printed by a press using various types of sheets of paper. That is, since the characteristic curves I and II shown in Fig. 1 change in accordance with the paper quality (including paper thickness, a color, and the like) of paper to be used, the optimum values of a light emission amount and a determination level cannot be kept constant. Therefore, in the above first and second methods, it is difficult to perform stable overlapping detection with high precision for sheets of paper having a wide range of paper quality. In addition, adjustment of the optimum values undesirably largely depends on the skills of an operator. These problems would also occur in the device of FR-A-2 492 349 which detects the overlapping of bank notes in a cash dispenser and relies on the fact that bank notes do not vary substantially in thickness and quality. In this device the transparency of the bank notes is detected by means of four pairs of photoelectric emitters and receivers placed on opposite sides of the bank notes. To determine whether bank notes overlap, the sum of the output of the four sensors is integrated over about 80% of the length of a bank note to determine a value N2 and the ratio of a value N1, determined by performing the same integration in the absence of a bank note, to N2 is calculated and compared with a predetermined maximum value. When this maximum value is exceeded it is determined that bank notes overlap. It is also essential for this device that all four photoelectric sensor pair operate correctly. In addition, in the conventional methods for sheet fed printing presses, overlapping of sheets of paper is detected by using a pair of light-emitting and light-receiving devices. Therefore, overlapping detection for sheets of paper having an extremely unstable fiber density (i.e. hungry sheets) is limited by using only the above methods to cause an erroneous operation. For this reason, when overlapping detection is not stably performed, an operator must stop the overlapping detecting function and perform visual inspection, resulting in large physical and mental burdens on the operator. Summary of the Invention It is, therefore an object of the present invention to provide a sheet overlapping detecting method which can perform stable overlapping detection for sheets of uneven density. According to a first aspect of the present invention, there is provided a sheet overlapping detecting method comprising the steps of radiating light emitted from first and second light-emitting means in a direction of thickness of a sheet to be fed, receiving light transmitted through the sheet to be fed by first and second light-receiving means, calculating a level difference between first and second output levels corresponding to received light amounts, and comparing the calculated level difference with a predetermined value, calculating a sum of the first and second output levels if the level difference is equal to or smaller than the predetermined value and setting a 1/2 value of the sum as effective data to be determined, setting a level value of a larger one of the first and second output levels as the effective data to be determined if the level difference is larger than the predetermined value, and comparing the effective data to be determined with a predetermined determination level, and detecting overlapping of sheets to be fed on the basis of the comparison result. According to a second aspect of the present invention, there is provided a sheet overlapping detecting method comprising the steps of radiating light emitted from first to Nth light-emitting means in a direction of thickness of a sheet to be fed, causing first to Nth light-receiving means to receive light transmitted through the sheet to be fed, setting an average of output levels, of first to Nth output levels corresponding to received light amounts, having differences with respect to a maximum level equal to or smaller than a predetermined value as effective data to be determined, comparing a predetermined determination level with the effective data to be determined, and detecting overlapping of sheets to be fed on the basis of the comparison result. Brief Description of the Drawings Fig. 1 is a graph showing a relationship between a light emission amount and an output level, which is different for one sheet of paper and two sheets of paper; Fig. 2 is a block diagram showing an arrangement of an apparatus according to the present invention; and Fig. 3 is a flow chart for explaining processing for obtaining effective data to be determined Dr in the apparatus shown in Fig. 2. Detailed Description of the Preferred Embodiments A sheet overlapping detecting method according to the present invention will be described in detail below. An embodiment of the present invention, which can reduce the influence of hungry sheets of paper will be described below with reference to Figs. 2 and 3. Fig. 2 shows an arrangement of an apparatus according to an embodiment of the present invention. Referring to Fig. 2, reference numerals 21-1 and 21-2 denote light-emitting devices; 22-1 and 22-2, light-receiving devices; 23-1 and 23-2, amplifiers for amplifying output electrical signals (analog signals) corresponding to received light amounts supplied from the light-receiving devices 22-1 and 22-2, respectively; 34, a multiplexer for selecting the amplified electrical signals supplied via the amplifiers 23-1 and 23-2 on the basis of a command (switching signal SX) from a microprocessor (to be referred to as a CPU hereinafter) 25; 24, an A/D converter for converting the amplified electrical signal selected by the multiplexer 34 into a digital signal and supplying the digital signal as a light reception signal of an output level Vin to the CPU 25; 26, a D/A converter for converting a light emission signal having a control level (digital signal) V out output from the CPU 25 into an analog signal; and 27, an amplifier for amplifying the analog signal output from the D/A converter 26 and supplies the amplified signal to the light-emitting devices 21-1 and 21-2. The CPU 25 is connected to keys 28 for inputting a command from an operator, a detection timing generator 29 for generating a sheet detection timing, and a monitor 30 for acknowledging processing information of the CPU 25 to an operator. A central processing system is constituted by the CPU 25, a ROM 31 for storing programs for operating the CPU 25, and a RAM 32 for storing/editing various types of information. Note that the light-emitting device 21-1 and the light-receiving device 22-1, and the light-emitting device 21-2 and the light-receiving device 22-2 are arranged as pairs to oppose each other with a front guide of a feeding table of a press (not shown) therebetween as described above in the explanation of the conventional apparatus. The amplification factors (gains) of the amplifiers 23-1, 23-2, and 27 are arbitrarily adjusted. An operation of the apparatus having the above arrangement will be described below. That is, in order to perform overlapping detection of a sheet 33 to be fed, the CPU 25 supplies a light emission signal having the control level V out to the D/A converter 26 beforehand, and an analog signal output from the D/A converter is amplified by the amplifier 7. The amplified signal is supplied to the light-emitting devices 21-1 and 21-2 to cause the light-emitting devices 21-1 and 21-2 to emit light in an optimum light emission amount. The light emitted from the light-emitting devices 21-1 and 21-2 is transmitted through the sheet 33, and the light transmitted through the sheet 33 is received by the light-receiving devices 22-1 and 22-2. When an operator inputs a detection command to the CPU 25 via the keys 28 or a detection command is supplied from the detection timing generator 29 to the CPU 25, the CPU 25 supplies a switching signal SX to the multiplexer 34. On the basis of the supplied switching signal SX, the multiplexer 34 selects the amplified electrical signal obtained via the amplifier 23-1, i.e., the amplified electrical signal corresponding to the received light amount of the light-receiving device 22-1. The selected amplified electrical signal is supplied to the A/D converter 24, converted into a digital signal, and fetched as a light reception signal having the output level V in by the CPU 25. The CPU 25 stores the fetched output level V in as D 1 in the RAM 32. On the basis of the supplied switching signal SX, the multiplexer 34 selects the amplified electrical signal obtained via the amplifier 23-2, i.e, the amplified electrical signal corresponding to the received light amount of the light-receiving device 22-2. The CPU 25 stores a light reception signal having the output level V in obtained from the selected amplified electrical signal in the RAM 32 as D 2 . The CPU 5 determines a larger one of the output levels D 1 and D 2 stored in the RAM 32 as D H and a smaller one, D L , and obtains a difference (level difference) between the output levels D H and D L . The CPU 25 compares the difference between D H and D L with a predetermined value Δd. In this case, Δd is set to be larger than a difference between D H and D L obtained by hungry sheets of paper. This Δd is obtained by experiments beforehand since it changes in accordance with the characteristics of light-emitting and light-receiving devices to be used. That is, even though the sheet 33 is hungry, if the hungry sheet is an ordinary one, a difference between D H and D L always becomes smaller than Δd ( D H - D L ≦ Δd ) provided that the light-emitting devices 21-1 and 21-2 normally operate. If a sheet is extraordinarily hungry, the sheet can be detected and removed. If one of the light-emitting devices 21-1 and 21-2 fails (burnout of a lamp or degradation), an output level obtained with respect to the fault light-emitting device is reduced. Therefore, the difference between D H and D L becomes larger than Δd ( D H - D L > Δd ). On the basis of the above concept, if D H - D L ≦ Δd , the CPU 25 determines that the light-emitting devices 21-1 and 21-2 normally operate and a hungry sheet is an ordinary one. The CPU 25 performs an arithmetic operation represented by the following equation to obtain effective data to be determined Dr: Dr = (D 1 + D 2 )/2 If D H - D L > Δd , the CPU 25 determines that one of the light-emitting devices 21-1 and 21-2 fails, removes the smaller output level D L , and sets the remaining output level D H as the effective data to be determined Dr ( Dr = D H ). The CPU 25 compares the effective data to be determined Dr obtained as described above with sheet overlapping detecting determination level Ds. If Ds > Dr, the CPU 25 determines that sheets 33 overlap. That is, in a normal state in which the light-emitting devices 21-1 and 21-2 normally operate and a hungry sheet is an ordinary one, an average value between the output level D 1 obtained for the light-emitting device 21-1 and the output level D 2 obtained for the light-emitting device 21-2 is set as the effective data to be determined Dr, and the sheet overlapping detecting determination level Ds is properly set. Therefore, even though the sheet 33 is hungry, if the hungry sheet is an ordinary one, overlapping detection can be stably performed. In an abnormal state in which one of the light-emitting devices 21-1 and 21-2 fails, the output level D L obtained for the fault light-emitting device is removed, and the output level D H obtained for a normal light-emitting device is set as the effective data to be determined Dr. Therefore, basic overlapping detection for sheets to be fed is continuously performed to prevent an increase in failure rate of the overlapping detecting apparatus, while a failure rate of the light-emitting and light-receiving devices is increased by using two pairs of devices. When both the light-emitting devices 21-1 and 21-2 fail, a relation of D H - D L ≦ Δd is obtained. Therefore, the above relation of Dr is applied to obtain the effective data to be determined Dr, and the overlapping detection operation is not ensured. In this case, however, since the effective data to be determined Dr becomes smaller than the sheet overlapping detecting determination level Ds, overlapping of sheets is constantly determined, resulting in a safe operation. In the above embodiment, two pairs of light-emitting and light-receiving devices are used. However, even if three or more pairs of devices are used, overlapping detection can be stably performed by the similar processing. Note that a system using two pairs of light-emitting and light-receiving devices is optimum in terms of a cost and an effect. A processing method to be executed when three or more pairs of light-emitting and light-receiving devices are to be used will be described in detail below. Assuming that the number of pairs is N, that a maximum level of N output levels D 1 to D N obtained for N pairs of light-emitting and light-receiving devices is D H , and that their minimum level is D L , a difference between the maximum and minimum levels D H and D L is obtained. If the level difference is equal to or smaller than Δd ( D H - D L ≦ Δd ), it is determined that a hungry sheet is an ordinary one and the N pairs of light-emitting and light-receiving devices normally operate, and an arithmetic operation represented by the following equation (2) is performed to obtain effective data to be determined Dr: Dr = (D 1 + D 2 + ··· + D N-1 + D N )/N If the level difference is larger than Δd ( D H - D L > Δd ), it is determined that at least one of N pairs of light-emitting and light-receiving devices fails, minimum levels D L at which differences between the maximum level D H and the minimum level D L of the N output levels D 1 and D N are equal to or small than Δd are removed, and an average value of the remaining output levels is set as the effective data to be determined Dr. Therefore, if the N pairs of light-emitting and light-receiving devices fail except for only one pair of light-emitting and light-receiving devices, an output level for the remaining light-emitting and light-receiving devices is set as the effective data to be determined Dr. In the above embodiment, when the difference between the maximum level D H and the minimum level D L is larger than the predetermined value, minimum levels D L at which the differences between the maximum level D H and the minimum level D L of the N output levels D 1 to D N are equal to or smaller than Δd are removed, and an average value of the remaining output levels is set as the effective data to be determined Dr. However, this processing may be modified such that the maximum level D H is extracted to obtain a difference between the maximum level D H and each of the output levels D 1 to D N and an average value of output levels having level differences equal to or smaller than Δd is set as the effective data to be determined Dr. Fig. 3 is a flow chart for explaining the above processing. Referring to Fig. 3, the output levels D 1 to D N are read out and stored in step 201. The maximum level D H is extracted from the stored output levels (step 202), and a difference (level difference) between the maximum level D H and each of the output levels D 1 to D N is calculated (step 203). It is checked whether each calculated level difference is equal to or smaller than Δd (step 204), output levels having level differences equal to or smaller than Δd are stored (step 205), and an average value of the stored output levels is set as the effective data to be determined Dr (step 206). That is, an average value of output levels having differences with respect to the maximum level D H equal to or smaller than Δd is set as the effective data to be determined Dr, and overlapping detection of sheets to be fed is performed on the basis of a comparison result between the effective data to be determined Dr and the determination level Ds (step 207). In each of the above embodiments, overlapping detection is performed for sheets to be fed to a press. However, the present invention is not limited to the above embodiments but can be practiced in various types of apparatuses requiring overlapping detection of sheets to be fed. As has been described above, according to the present invention, a level difference between output levels from two light-receiving devices is compared with a predetermined value. If the level difference is equal to or smaller than the predetermined value, a 1/2 value of a sum of the two output levels is set as a determination level. If the level difference is larger than the predetermined value, a larger one of the two output levels is set as the determination level. Furthermore, of output levels from N light-receiving means, an average of output levels having level differences with respect to a maximum output level equal to or smaller than a predetermined value is set as the determination level. Therefore, stable overlapping detection can be performed for a hungry sheet of paper. In addition, even though one or more light-emitting devices fail, overlapping detection of sheets to be fed can be performed if at least one pair of light-emitting and light-receiving devices normally operate, thereby preventing an increase in failure rate as the overlapping detecting apparatus.

Claims: A sheet overlapping detecting method comprising the steps of: radiating light emitted from a plurality of light-emitting means (21-1, 21-2) in the direction of a sheet of paper to be fed (33); receiving light transmitted through said sheet (33) with first a plurality of light-receiving means(22-1,22-2); characterized in that it further comprises the steps of: determining the maximum output level of said plurality of light receiving means (22-1,22-2) corresponding to received light intensities; calculating a difference value associated with each of the remaining output levels corresponding to the difference between said remaining output level and said maximum output level and comparing each said difference value with a predetermined value ( d); and calculating the average value of the maximum output level and the remaining output levels having a difference value less than or equal to the predetermined value ( d) and setting the effective data to be determined as said average value; and comparing the effective data to be determined (D r ) with a predetermined determination level (D s ), and detecting overlapping sheets on the basis of the comparison result. A sheet overlapping detecting method according to claim 1, characterized by the step that when all the difference values associated with the remaining output levels exceed the predetermined level ( d), the effective data to be determined (D r ) is set as the maximum output level.

Inventors: KURATA YOSHIAKI, KURATA, YOSHIAKI
Applicants: KOMORI PRINTING MACH, KOMORI CORPORATION
Application year: 1990
Main classification: B65H7
IPC codes: B65H7