JPH0354628B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of Industrial Application The present invention relates to a printing defect detection device capable of detecting printing defects in a wide variety of printed matter.

2. Description of the Related Art In general, a printing defect detection device is configured to convert the state of reflected light from a printed surface into an electrical signal, and compare this with an electrical signal from the paper surface to be detected to determine the quality. The following devices have been proposed as this type of device.

(1) A static device in which optical sensors are installed at several points on the paper, and the signals are taken while both the paper and the sensors are stationary during judgment.

(2) A static device that captures an entire image of the printed surface, like a television camera.

(3) A dynamic device that moves the sensor or paper surface to extract changes in reflected light within a certain zone of the paper surface as a signal.

Problems to be Solved by the Invention In such conventional devices, the sensitivity of the sensor must have a considerable range when operating the comparison circuit, and depending on the type of printed material, the detection sensitivity may be insufficient. There will be things that cannot be determined.

Furthermore, conventional methods have the disadvantage that printing defects cannot be detected appropriately because they are directly influenced by the sensitivity characteristics of the sensor.

OBJECTS OF THE INVENTION It is therefore an object of the present invention to minimize such inconveniences, provide flexibility depending on the type of printed material, and increase detection ability.

Means for Solving the Problems The present invention irradiates light from a light emitting element onto a printing surface,
The reflected light is received by a light receiving element and converted into an electrical signal.The electrical signal obtained from the test printing surface and the electrical signal obtained from the standard printing surface are compared by a comparator, and the comparison result is sent to the allowance setting device. It is determined as an error when the specified allowance setting value is exceeded.
The judgment result is used to detect the quality of the printed surface, and furthermore, a density sensitivity corrector is provided in front of the comparator, and a correction curve determined by the black and white level of the printed surface and the output electric signal level is used. The gist of the present invention is a printing defect detection device characterized in that it is configured to correct the density sensitivity characteristics of a light receiving element.

Effects of the Invention According to the present invention, since the gray scale sensitivity corrector is provided, settings can be made to correct the discrepancy between the comparison of the detection device and the comparison of human vision. Correction of density sensitivity can be set arbitrarily to accommodate differences in printed matter (paper, metal, etc.), differences in screen shape, density structure, etc.

Furthermore, the density sensitivity of the light receiving element to the printing surface can be corrected so as to approach the visual sensitivity characteristic. Therefore, depending on the detection target, printing defects can be detected in the same way as visual inspection.

Embodiment Now, to briefly describe an embodiment of the printing defect detection device of the present invention, a large number of measurement points are created, for example, in a grid pattern on the entire printing surface so as to detect the entire printing surface. At that time, a large number of light receiving elements are arranged across the entire width of the printed surface, and the printed surface is moved to time-divide and convert the continuous analog electrical signals emitted by each light receiving element into digital electrical signals, making it possible to perform a large number of measurements. Obtain digital electrical signals corresponding to the points.

At each measurement point, each light-receiving element receives reflected light from the printed surface, and at this time, each light-receiving element receives the light in an overlapping manner.
The degree of overlap can be adjusted by adjusting the arrangement interval of the light receiving elements, the shape of the lens, the method of time division, etc.

The comparison at each measurement point was simply made as E ₁ −E ₁ ′=0 (E ₁ is the signal of the standard printing surface measured at one point, and E ₁ ′ is the signal of the detected printing surface). , there is a risk that slight unevenness in the printing surface, errors in measurement points, deviations in measurement points, etc. may all be detected as printing defects. Therefore, E ₁ −E ₁ ′<±M. M is an allowance (tolerable error) that is thought to occur when measuring the amount of reflected light or changing the ink density at the same measurement point. It is determined that an error point has occurred only when this allowance M is exceeded.

Such allowances M include detection width allowance, ink density allowance, point limit allowance, error limit allowance, density sensitivity allowance, etc., and multiple allowances can be set separately (or all at once) depending on the type of printing, etc. Set the allowance to an arbitrary setting value in advance. Allowance settings can also be automated.

Since the sensitivity characteristics of the light-receiving element and the sensitivity characteristics of humans are different, in order to get closer to human failure judgment (although this may not always be the case),
Correct the density sensitivity.

Depending on the type of printing, the points that become error points are further aggregated and calculated as the ratio of the total number of error points to the number of all measurement points, that is, the defective ratio (this is treated as the number of error points among all measurement points). (also good). Then, when it exceeds a preset failure ratio setting value, it is determined that the detected printing surface is defective.

Depending on the type of printing, in order to increase detection accuracy, it may be necessary to determine a printing error if even one error point appears. Generally, the defective ratio is
Set arbitrarily between 0.1 and 100%.

Note that it would be convenient to be able to set the pure black level, pure white level, print size, etc. in advance.

Embodiments of the present invention will be described in detail below with reference to the drawings.

First, to explain the light emitting element and light receiving element of the sensor, as shown in FIG. 10b, a blower duct 10c that is provided along the entire sensor 10a and cools the sensor 10a by applying air to keep the temperature constant; Tube 1 for supplying
0e, a drive unit 10f, and a support arm 10g.

As shown in FIG. 1B, the aforementioned sensor section 10
A has a frame 11, and a large rectangular opening 11a is formed in the upper central part of the frame 11, and a transparent plate G is attached to the opening 11a. This plate G is preferably made of transparent acrylic or transparent glass. The outer surface of the plate G has a diffused reflection shape, and the inner surface of the plate G has a mirror finish.

The frame 11 is made of a material that blocks light from the outside. Of course, the frame 11 may be constructed in various other shapes.

The lens 13 is preferably made of transparent acrylic or glass and has a mirror finish. The lens 13 is rod-shaped and typically has a circular cross section. Depending on the situation, the cross-sectional shape of the lens 13 may be oval, egg-shaped, semicircular, etc.

The lens 13 for the light receiving element 18 is mainly for adjusting the field of view, and the lens 13 for the light emitting element 21 is mainly for adjusting the level.
Therefore, the lens for the light receiving element 18 and the lens for the light emitting element 21 can have different cross-sectional shapes.

In the illustrated example, the support 14 is a rectangular parallelepiped block, and has three through holes 14a formed therein, into which the lens 13 is passed. The support 14 thereby supports the three lenses 13.

The printed board 20 is an example of a board, and a normal electric circuit board can be used. Print board 2
A plurality of light-receiving elements 18 are fixed in a row at regular intervals in the center of the photodetector 0 .

For example, these light receiving elements 18 are arranged at a distance of 10 to 20 mm. Such an arrangement provides high resolution. It is also possible to arrange the light receiving elements at narrower intervals, but in that case, there is a possibility that the cost will increase unnecessarily although the separation power will not improve much. Of course, the probability of a detection error or discrimination error occurring varies depending on the type of printed surface to be measured, and cannot be definitively determined.

If the entire length of the frame 11 is made integral and the printed board 20 is made by combining a plurality of small units, the versatility will be increased, especially regarding the detection width.

Furthermore, the length of the sensor can be made variable by making the entire frame and other parts into a unit and connecting a plurality of units. In that case, each unit becomes compatible.

The light receiving element 18 can be a photodiode, for example, a photodiode AFT1O01W manufactured by Siemens. Since this photodiode has a built-in amplifier, it has the characteristic of being resistant to noise in the detection signal, and is also of a flat type, which has the advantage of minimizing the installation volume.

Light-emitting elements 21 are arranged in pairs at equal distances on both sides of each of the light-receiving elements 18.
In the illustrated example, four light emitting elements 21 are arranged on each light receiving element 18. The light emitting element 21 can be composed of a light emitting diode, and in order to save installation space, it is preferable to use a flat type light emitting diode, as shown in the illustrated example, without a light bulb or lens on the outside.

Further, it is preferable to use a non-directional light emitting element 21 in order to reduce variations in the optical axis and the amount of light.

When the light-emitting element 21 is yellow-green, more colors can be detected on the printed surface, which increases the practical merit.

Light emitting diodes also have excellent durability and stability.

The printed board 20 is provided with necessary electrical wiring, and the light receiving element 18, the light emitting element 21, a resistor (not shown), etc. are electrically connected. Various types of wiring can be placed in the space below the printed board 20 as required.

It is preferable that each light-emitting element 18 incorporate an integrated IC amplifier, such as the photodiode manufactured by Siemens, described above, so that the electric signal is amplified for each light-receiving element 18 before noise is introduced.

The sensor 10 has a temperature compensation mechanism (for example,
57-207744) is preferably provided.

Other embodiments of the sensor used in the present invention will be described with reference to FIGS. 4 to 6.

The printed surface monitoring sensor 10 according to this embodiment also has the overall configuration shown in FIG. 1A, and has a sensor section 1a.
However, frame 11, holder 12, transparent plate G
(a lens 13 is integrally formed in a part thereof), a light receiving element 18, a light emitting element 21, a printed board 20, and the like.

As clearly shown in FIG. 5, the frame 11 has a large opening 11a formed in its upper center. The frame 11 is made of a material that blocks light from the outside.

The holder 12 is shaped to accommodate the frame 11 therein. (The frame 11 includes the holder 12 in a broad sense.) The holder 12 is adapted to be slidably mounted within the frame 11 in its longitudinal direction. A folded piece 12a facing inward is formed on the upper edge of the holder 12.

The transparent plate G is attached to the opening 11a of the frame 11. It is preferable that the plate G is made of transparent acrylic or transparent glass. When the outer surface of the plate G has a diffused reflection shape, stable detection is possible even on a glossy printed surface. A lens 13 is integrally formed on the inner surface of the plate G. The surface of this lens 13 is given a mirror finish. The width of the plate G is the same as that of the frame 11, and the plate G is held integrally with the frame 11 by a folded piece 12a at the upper edge of the holder 12.

The printed board 20 is an example of a board, and a normal electric circuit board can be used. Print board 2
A plurality of light-receiving elements 18 are fixed in a row at regular intervals in the center of the photodetector 0 . These light receiving elements 18 are arranged at a distance of 10 mm from each other when the diameter of the field of view is 15 mm.

Light-emitting elements 21 are arranged in pairs at equal distances on both sides of each of the light-receiving elements 18.
The light emitting element 21 can be composed of a light emitting diode, and is preferably a small light emitting diode in order to save installation space. In the illustrated example, a hole 20a is formed in the printed board 20, and the light emitting element 21 is fixed in a slightly embedded manner.

One light receiving element 18 and light emitting elements 21 on both sides thereof
A large number of sets are arranged in a line (this is used in a broad sense and includes staggered patterns, etc.).

The printed board 20 is provided with necessary electrical wiring, and the light receiving element 18, the light emitting element 21, etc. are electrically connected to terminals of a connector (not shown), respectively.

Next, a detailed description will be given of how to process the electrical signal obtained by the sensor 10 as described above to determine a printing error.

In FIG. 7, the sensor 10 is fixed and the printing surface A is moved in the direction of the arrow B. In FIG. (Conversely, it is also possible to move the sensor 10 and make the printing surface A stand still.The present invention may be used in either manner, and the movement of the printing surface A in this specification includes both modes, and the movement of the printing surface A relative to the sensor 10 is (say something)

Then, sensor 1 crosses the entire width of printing surface A.
0 is arranged, a large number of light emitting elements 21 and light receiving elements 18 are present over the entire width of the printing surface A, and as illustrated in FIG.
A large number of measurement points (pixels) are arranged like a checkerboard over the entire print surface A. Note that, in reality, the measurement points naturally overlap each other by a considerable width in the vertical and horizontal directions due to the characteristics of reflected light.

To simplify the illustration, only one light-receiving element 18 is shown in FIG. 7, and one zone consisting of a series of measurement points is correspondingly hatched in FIG. 14.

Now, as the printing surface A moves, the print content (that is, the amount of light received) on the printing surface A captured by the light receiving element 18
changes. In other words, each light-receiving element 18 exhibits a change in the amount of light received depending on the print content. The amount of light received is converted into an analog electrical signal by the light receiving element 18. This analog electrical signal is amplified by an amplifier D, then enters an A/D converter E, where it is converted into a digital electrical signal. A/D converter E performs sampling at a constant time using a signal from clock circuit Q (or encoder). In other words, it is time-divided. Therefore, if the printing surface A is moved at a constant speed, digital electrical signals of measurement points at regular intervals are output one after another along the moving direction of the printing surface A.

FIG. 8 shows how the printing surface A is moved and one light-receiving element 18 is used from the front end to the rear end of the printing surface A.
An example of an analog electrical signal and a digital electrical signal of one zone (corresponding to the hatched area in FIG. 14) when measured by .

The digital electric signal is sent from the A/D converter E to the gray scale sensitivity corrector P, where the gray scale sensitivity is corrected and then stored in the first memory F and the second memory C.

The shading sensitivity corrector P corrects the discrepancy between the comparison of the input value by the sensor 10 and the comparison by human vision, and it compensates for the difference in the material of the printed matter (paper, metal, etc.), screen recording, and other shading configurations. In order to accommodate the differences, a large number of correction characteristics can be arbitrarily set depending on the intended use.

The gray scale sensitivity corrector P will be explained with reference to FIG. 16. The vertical axis shows the black and white level (that is, the amount of received light), and the horizontal axis shows the electrical signal (voltage in this example). The line R shows the density sensitivity characteristic of the light receiving element 18 (this is ideal, but there are errors in reality), and the black and white level and the voltage are directly proportional and form a straight line. However, since human shading sensitivity characteristics vary depending on the type of printing and other factors, they are usually different from the shading sensitivity characteristics of the light receiving element 18. Therefore, for example, correction curves S1 to S4
A gradation sensitivity corrector P is provided to correct the gradation sensitivity characteristics as shown in FIG. Taking the correction curve S3 as an example, between T1 and T2, a positive correction is made to the line R of the sensitivity characteristic of the light receiving element 18, and T
Negative correction is performed between T2 and T3 so that the difference in shading appears as a larger voltage difference in the intermediate region between black and white levels. In other words, the correction curve S3 is suitable for detecting printing defects with high sensitivity in the intermediate density region.

The correction curves S1 and S4 are suitable for detecting printing defects with high sensitivity in dark and light areas close to black and white, respectively. The correction curve S2 is suitable for detecting printing defects with high sensitivity in dark and light areas close to both black and white.

Correcting the gray scale sensitivity has the same effect over time as changing the allowance setting value according to a large gray level (black and white level). By connecting the shading sensitivity corrector to the allowance setting device, you can change the allowance width between, for example, a level close to black (white blank areas), a level close to white (blank paper areas), and an intermediate level between them (general printing areas). It is possible.

After correcting the density sensitivity in this manner, the digital electrical signals are stored in the first memory F and the second memory C, respectively, and the corresponding digital electrical signals are extracted and input into the arithmetic circuit H. This arithmetic circuit H extracts the absolute value of the difference between the corresponding digital electric signals of the first memory F and the second memory C as a voltage difference. This voltage difference is compared in a comparator circuit J based on the allowance setting value from the allowance setting device I, in which an arbitrary allowance is set in advance according to the printing content.
Digital electrical signal of memory F and second memory C
If the difference from the digital electric signal is greater than or equal to the allowance setting value, it is determined that it is an error point. When it is determined that it is an error point, only the result of failure is entered into the counter K. If it is good, it is not counted.

The allowances to be set include detection width allowance, ink density allowance, point limit allowance, error limit allowance, and others.

Regarding the detection width allowance, an allowance is set above and below a reference value, and when the above and below set values are exceeded, it is determined that it is an error point.

The ink density allowance is an allowance for changes in ink density. Particularly when updating the reference value, there is a possibility that printing exceeding the desired density limit may occur, so it is desirable to set the ink density allowance independently of the detection width allowance.

The point limit allowance is the allowance for the displacement of the printing surface A in the front-rear direction with respect to the sensor 10. This point limit allowance is set when the accuracy of the printing material feeding mechanism is low. If the set value of the point limit allowance is large, the detection judgment will be lenient.

The error limit allowance is the allowance for the deviation of the printing surface A in the left-right (horizontal) direction with respect to the sensor 10. The setting value is determined particularly in relation to detection accuracy.

For example, as shown in FIG. 15, detection width allowances are set above and below the electrical signal 200 of the reference value, and the electrical signals 201 and 2 of the detection width allowance set value are
02, and electrical signals 203 and 204 of ink density allowance setting values are set above and below independently of this. After that, the electrical signal 205 obtained by detecting and measuring the printed surface is transmitted to the measurement point SE.
If the detection width allowance setting value 202 is exceeded at the measurement point SG, and the ink density allowance setting value 204 is exceeded at the measurement point SG, an error point is generated in either case.

For example, between the pure black level and the pure white level.
The contents of the first memory F and the second memory are divided into 256 stages.
The contents of memory C are compared to determine the difference.

Perform these operations in the same order for all the digital electrical signals stored in the first memory F and second memory C, and perform one operation from the beginning to the end of the printing surface A.
The measurement points are compared, the second measurement point is compared, the third measurement point is compared, and so on until the mth measurement point (the last measurement point). Then, the number of error points is counted by a counter K.

When the contents of the first memory F and the second memory C are completely compared, the data of the counter K is input to the arithmetic circuit L. Then, the percentage of the total number of measurement points (m pieces) and the actual count number of the counter K is calculated.

A percentage ratio as a judgment standard is set in advance by the defect ratio setting machine U, and the calculation result of the calculation circuit L is compared with the preset percentage ratio (defective ratio), and finally the printed surface is determined. It is determined whether the overall printing is defective or not.

The failure ratio setting device U may be configured to be able to set all values with an error point count of 1 or more, but normally the failure ratio is set arbitrarily within the range of 0.1 to 100%.

In the present invention, the above-mentioned comparative judgment is performed for each of a large number of light receiving elements 18 arranged across the entire width of the printing surface A across the moving direction of the printing surface A.

Still another embodiment of the present invention will be described with reference to FIGS. 9 to 13.

In FIG. 9, the vertical axis indicates the black and white level of the printing surface, that is, the amount of reflected light. Line X indicates the level of the pure white printed surface, and line Y indicates the level of the pure black printed surface. In addition, A, B, H... are encoder 84
(FIGS. 12 and 13) shows the positions of a series of a plurality of measurement points along the moving direction of the printing surface A by one light receiving element 18. Each light receiving element 18
Since similar measurement points are taken for each print surface A, a large number of measurement points are distributed over the entire print surface A in an overlapping manner vertically and horizontally at regular intervals as shown in FIG.

For convenience of explanation, only one light receiving element 18 will be described. In FIG. 9, signal 100 represents an analog electrical signal of a standard printing surface;
a is an analog electrical signal obtained by adding level allowance M′ to signal 100, and signal 100b is signal 1
It is an analog electrical signal obtained by subtracting the level allowance M' from 00. The detected printed surface is measured and the comparison judgment described above is performed to determine the signal 100a and the signal 100b.
If no measurement point falls between the two, it is determined that it is an error point.

Now, even though the detected printing surface is good printing, the amount of reflected light may vary due to differences in the material of the printing surface, and it may be determined as an error point. To avoid such judgment errors,
A correction level α is added or subtracted from E ₁ ′ to obtain E ₁ −(E ₁ ′±α)<±M, and then comparison is performed to correct the difference in the amount of reflected light and make a determination.

For example, when the paper quality of the detected printing surface A is dark, even if the printed content is exactly the same, the signal 101 of the detected printing surface approaches the line Y as a whole due to the influence of the paper quality. If signal 101 is compared with signal 100 as it is, there is a possibility that the detected printing surface will be judged as a printing error, so signal 101 is set to correction level α.
is added to make the signal 101' and then compared.

This level allowance is mainly due to paper quality,
This is to deal with printing unevenness and sensor drift, but the point allowance explained next mainly affects the encoder 84 (Figs. 12 and 13) and the printing surface A.
This is to accommodate the discrepancy between the two.

Regarding the detection width allowance of one measurement point, the level allowance is the allowance (tolerable value) in the vertical direction, and the point allowance is the allowance (tolerable value) in the horizontal direction.

If the level allowance and point allowance are set to exactly the same value, the allowable range will be a square centered on the reference value. Of course, level allowances and point allowances can be made independent of each other. In that case, the two may have different values. In that case, the permissible range is a rectangle.

Now, in either case of level allowance alone (FIG. 9) or a combination of level allowance and point allowance (FIG. 10), if the sampled data exceeds the plus or minus allowance setting value, it is determined as an error point. Then, use one (or both) of the following methods to determine a printing error.

Judgment method (1): If the number of error points among all measurement points exceeds a set value (this corresponds to the defective ratio set value), it is determined that there is a printing error.

Judgment method (2): First, a place where error points are consecutive is defined as an error block, and when the next calculated value exceeds the defective ratio setting value, it is judged as a printing error.

L+S/PL L is the peak value of the error block S is the sum of the levels of the error block P is the number of error points of the error block Next, a more specific embodiment of the method of the present invention will be explained with reference to FIG. .

The sensor 10 has the configuration as already described.
It is provided with light receiving elements 18 of No.1...No.n.

The sensor 10 is made to reflect light from each light emitting element 21 onto the printing surface A, and receive the reflected light with each light receiving element 18 and convert it into an analog quantity of electricity.

The output of each light receiving element 18 is amplified by an amplifier 72. The amplifier 72 is preferably a buffer amplifier that performs impedance conversion in order to prevent noise from entering.

The amplified analog electrical signal is sent to AD converter 71. Although it is preferable from the viewpoint of detection speed to provide various components after the AD converter 71 for each light receiving element 18, in the case of low-speed printing, etc., a multiplexer 79 may be provided and used together.

The AD converter 71 samples analog electrical signals and converts them into digital electrical signals in response to measurement pulses from the measurement time controller 82. The measurement time controller 82 is activated by signal pulses from a starter 83. The starter 83 is attached to the power shaft of the printing machine, and is designed to generate a signal pulse at the same time as printing starts. Sampling is performed in a time division manner using pulses from encoder 84. Encoder 84 can be attached to the power shaft of the printing machine.

In the AD converter 71, the analog signal is converted into a digital signal, and the digital signal is sent to the gradation sensitivity corrector 73, where the gradation sensitivity correction is performed as already explained with reference to FIG. Memory controller 74
According to the instructions, the measurement points are stored in the storage circuit 75 for each measurement point.

The memory circuit 75 can be broadly divided into a standard memory 75a and a detection memory 75b depending on its role. Standard memory 75a and detection memory 75
b has no functional difference. Standard memory 75a
Detection memory 7 stores information on the standard printing surface.
5b stores information on the detected printed surface.

The contents of the memory 75b are compared with the contents of the memory 75a by an arithmetic unit 81, and any difference greater than the allowance setting value is determined as an error point. That is, the information (hereinafter referred to as E ₁ ) of the first measurement point on the standard printing surface in the memory 75a is derived to the computing unit 81. Next, memory 75b
Information on the first measurement point on the detected printed surface (hereinafter referred to as E ₁ ') is derived to the computing unit 81. Then, the arithmetic unit 81 determines whether E ₁ ' is a defective point by determining that the absolute value of (E ₁ -E ₁ ')<M.

Among the setting values of the allowance M, the setting value of the detection width allowance is arbitrarily set by the detection width allowance setter 90. In this case, the level allowance and point allowance may be set to the same value, but they may also be set to different values.

The set value of the ink density allowance is arbitrarily set by the ink density allowance setter 92.

When an error point signal is output, it is possible to immediately determine that there is a printing defect on the printed surface, but in order to make the determination more flexible, one pulse is sent from the arithmetic unit 81 to the failure rate detector 51. Defective rate detector 5
1 counts the number of pulses.

Next, the information on the second measurement point on the standard printing surface (E ₂ ) and the information on the second measurement point on the detection printing surface (E ₂ ′) are compared in the same way,
It is determined whether E ₂ ' is an error point signal.

This operation is performed for all measurement points.

When all measurement points have been compared in this manner, the defective rate detector 51 stores the number of error point signals. Whether printing is defective or not is determined based on the ratio of the number of error point signals to the total number of measurement points. In other words, the determination is made based on the number of error points among all points.

The defect ratio setting device 42 arbitrarily sets the ratio of the error point signal to the total number of measurement points, that is, the defect ratio. For example, when the defective ratio setting device 42 is set to 0.2% and the total number of measurement points is 1000, the limit for the number of error points is 2 points. These two pieces of information are used by the defective rate setting device 42.
from there to the defective rate detector 51. Then, the number of error point signals and the information sent from the defective rate setting device 42 are compared.

In addition, the judgment result of printing defects is usually displayed on a display (not shown) or an alarm (not shown) is displayed.
to issue a warning. If necessary, printed matter determined to be defective can be automatically ejected.

Setting of the ink density allowance setting value in the ink density allowance setting device 92, setting of the detection width allowance setting value in the detection width allowance setting device 90, and setting of the defective rate in the defective rate setting device 42 can also be performed automatically.

In addition, as shown in FIG.
2 and the defective rate detector 51 may be omitted, and instead, an arithmetic unit 50 for calculating the above-mentioned value of (L+S/P) may be separately provided to perform the determination method (2). Except for the arithmetic unit 50, the embodiment of FIG. 13 has the same configuration as the embodiment of FIG. 12.

In order to be compatible with high-speed printing presses, the sampling time at each measurement point must be kept as short as possible, but if the above-mentioned allowance (tolerable value) is stored in memory for all points, it is possible to In this case, the processing of samples, comparisons, and judgments becomes faster, and the processing speed is improved.

[Brief explanation of drawings]

FIG. 1A is a perspective view showing the appearance of a printed surface detection sensor according to the present invention. FIG. 1B is a plan view schematically showing a part of the printed surface detection sensor according to the present invention. FIG. 2 is a sectional view taken along the line X--X in FIG. 1. FIG. 3 is a sectional view taken along line Y--Y in FIG. 1. FIG. 4 is a perspective view showing an example of a printed surface monitoring sensor according to the present invention. FIG. 5 is an exploded view of the print surface monitoring sensor shown in FIG. 1. FIG. 6 is a sectional view of the print surface monitoring sensor shown in FIG. 1. FIG. 7 is a block diagram for explaining the principle of the method of the present invention. FIG. 8 is a waveform diagram showing the flow of the detection signal and the level at each detection point. FIG. 9 is an explanatory diagram of the principle of the printing error detection method of the present invention. FIG. 10 is a diagram showing the relationship between the reference value, level allowance, and point allowance. FIG. 11 is a diagram showing an error block. FIGS. 12 and 13 are block diagrams showing other embodiments of the printing error detection method of the present invention. FIG. 14 is a diagram showing the relationship among the printed surface, the light receiving element, and the measurement point. FIG. 15 is an explanatory diagram showing the interrelationship among the reference value, ink density allowance, and detection width allowance. FIG. 16 is a diagram showing the principle of density sensitivity correction. 10...Sensor, 11...Frame, 12...
...Holder, 13...Lens plate, 14,1
5, 17...Support, 16...Base, 18...
...Photodetector, 20...Printed board, 21...
Light emitting element, 72...Amplifier, 73...AD converter, 74...Memory controller, 75
a...Standard memory, 75b...Detection memory,
84... Encoder, 42... Defective rate setting device,
50... Arithmetic unit, 51... Defective rate detector, A...
Printed surface, D...Amplifier, E...A/D converter, F...First memory, C...Second memory,
H...Arithmetic circuit, I...Allowance setting device, J...
...Comparison circuit, K...Counter, L...Arithmetic circuit,
P...shade sensitivity corrector, Q...clock, U...
Defective ratio setting device.

Claims (1)

[Claims] 1. Light is irradiated from the light-emitting element to the printing surface, the reflected light is received by the light-receiving element and converted into an electrical signal, and the electrical signal obtained from the test printing surface and the electrical signal obtained from the standard printing surface are compared by a comparator. When the comparison result exceeds a predetermined allowance setting value defined by the allowance setting device, it is determined as an error, and the determination result is used to detect the quality of the printed surface. A density sensitivity corrector is installed in front of the
A printing defect detection device characterized in that it is configured to correct the density sensitivity characteristics of a light receiving element along a correction curve determined by a black and white level of a printed surface and an output electric signal level.