GB2258039A

GB2258039A - Controlling the print quality of printed products

Info

Publication number: GB2258039A
Application number: GB9209496A
Authority: GB
Inventors: Tobias Muller
Original assignee: Heidelberger Druckmaschinen AG
Current assignee: Heidelberger Druckmaschinen AG
Priority date: 1991-05-04
Filing date: 1992-05-01
Publication date: 1993-01-27
Also published as: JPH05208488A; FR2676392A1; GB9209496D0

Abstract

The invention relates to a device for checking the print quality of printed products from a printing machine, with a first optical sensor supplying real-value data for scanning the print image of the printed product, and with a comparator which compares the real-value data with allocated reference data and produces a discrimination signal in the event of inadmissible deviations between the real-value data and the reference data. According to the invention, this device is characterised in that the reference data originate from a second optical sensor (8) which, synchronously with the first optical sensor (4) scans a reference image (9) corresponding to the print image, in that the comparator (6) undertakes a real-time processing of the data flows supplied by the two sensors (4, 8). The invention also proposes a corresponding method. <IMAGE>

Description

Device and method for controlling the print quality of printed products The invention concerns a device for controlling the print quality of printed products of a printing machine, with a first optical sensor, that supplies actual-value data, for scanning of the printed image of the printed product, and with a comparator that compares the actual-value data with preset specified-value data and that generates a rejection signal in the case of unacceptable differences between the actual-value data and the specified-value data.

To control the print quality of printed products of a printing machine, it is necessary to compare the print result with a reference at regular intervals. If the differences between the printed image to be controlled and the reference are located within a permitted tolerance, the printed product can be accepted as error-free. The comparison can be performed by the printer itself, but it is preferably effected by an independently operating technical device. This device contains a sensor for scanning of the printed image of the printed product that transfers its actual-value data to a comparator. In addition, the comparator receives preset specified-value data of the reference. In the case of unacceptable differences between the actual-value data and the specified-value data, a rejection signal is generated, i.e. the printed product is a misprint.

From DE-OS 29 21 862, a machine for the automatic quality control of freshly-printed bank notes and securities is known. With this machine, quality control is effected by scanning the print result with a sensor that supplies actual-value data, whereby the actual-value data is compared with specified-value data stored in an electronic memory.

The specified-value data is reference data.

From DE-OS 25 53 721, a method for determination of the shift of an image is known. The image is optically scanned, whereby actual-value data is generated. This actual-value data is compared with specified-value data which originates from an electronic memory.

In a method for optical surface analysis made known by DE-OS 33 14 465, an actual image with a multitude of scanning spots is optoelectronically scanned and compared with stored reference-image data. The reference-value data is stored as software.

Finally, in a method and an arrangement for image comparison made known by DE-OS 37 14 011, actual-value data is determined by a sensor and specified-value data originates from a data record. By comparison of the actual-value data with the specified-value data, information concerning defects can be supplied.

From the state of the art described above, it is clear that electronic memories are used to supply the reference image.

The storage space required for adequate quality control is so extensive that such a semiconductor memory is extremely expensive. In a known method, therefore, the information is considerably reduced before the specified-value data of the reference is stored, so that only a correspondingly smaller number of storage locations is required. However, the information reduction results in the lowering of the quality control requirements.

In addition, electronic storage of the specified-value data of the reference entails a corresponding calculation time, which means that rapid memory-access methods are required for quick processing, which, in turn, involves corresponding costs.

The invention is therefore based on the technical problem of creating a device of the type described above which permits high-quality control of print quality by means of a simple and inexpensive arrangement.

According to the invention, this problem is solved by an arrangement in which the specified-value data originates from a second optical sensor that, in synchronism with the first optical sensor, scans a reference print corresponding with the printed image, and in which the comparator performs real-time processing of the data flows supplied by the two sensors. Thus, in contrast with the objects of the prior art, the specified-value data is not stored in an electronic memory but is generated by an optical sensor which scans the printed image of a reference print during print-quality control. At the same time, the printed image of the printed product is synchronously scanned with a corresponding optical sensor, so that two data flows are produced.One data flow consists of the actual-value data of the first sensor, that scans the printed product, and the other data flow consists of specified-value data which originates from the sensor that scans the reference print. According to the invention, the comparator performs real-time processing of the data flows that are supplied simultaneously, whereby according to the desired resolution and the large quantity of data this requires - a print-quality control of a correspondingly high quality can be performed. Expensive electronic memory devices for the reference data are therefore not necessary. Instead of reference data that is stored in electronic data memories, a reference-image storage according to the invention is used, i.e. a storage that contains an actual printed image of an error-free specimen as a reference.

According to a development of the invention, the performance of line scanning and/or column scanning by the sensors is envisaged. The entire surface of the printed image of the printed product, or of the reference print, is not scanned all at once; instead, raster scanning is performed. For proper functioning, it is necessary that the two sensors perform synchronous scanning. For this, a suitable control device is provided.

The second sensor is preferably located in a device for supplying of a reference print that is separate from the printing machine The printed sheet constituting the reference is supplied in this device and scanned by the second sensor that supplies the specified-value data. The comparator can also be located in the device for supplying of a reference print.

Alternatively, however, the second sensor can also be located in a device for supplying of the reference print that is positioned inside the printing machine. The printing machine itself then contains the equipment for printquality control.

The first sensor is preferably positioned inside the printing machine in such a way that it can scan the printed image of the printed product while it remains in the normal print-conveyance cycle. This has the advantage that, in order to carry out print-quality control, it is only necessary to interrupt the printing process, whereby, however, the fresh printed product to be examined remains in the print-conveyance cycle, i.e. it need not be removed.

When the quality control has been concluded, the examined printed product - as is usual in production printing - can then be withdrawn; it thus remains - in so far as the printed product is acceptable - within the product cycle.

In addition, the invention concerns a method for controlling the print quality of printed products of a printing machine, with a first optical sensor, that supplies actual-value data, for scanning of the printed image of the printed product, and with a comparator that compares the actualvalue data with defined specified-value data and which generates a rejection signal in the case of unacceptable differences between actual-value data and specified-value data, whereby the specified-value data originates from a second optical sensor that scans, in synchronism with the first optical sensor, a reference print corresponding to the printed image, and whereby the comparator performs real-time processing of the data flows supplied by the two sensors.

The drawings illustrate the invention on the basis of two examples. The following is shown: Figure 1 a schematic representation of the device according to a first example and Figure 2 a schematic representation of the device according to a second example.

Fig 1 shows a printing machine 1 that is designed as a sheet-fed offset printing machine. The printing machine 1 features several printing units 2. A device 3 for printquality control is allocated to the last printing unit 2. A first optical sensor 4 is positioned in the region of the print-conveyance path of the last printing unit 2. During a printing stop, this first optical sensor 4 can perform line scanning and/or column scanning of the printed image of the corresponding freshly-printed product. With this scanning, the first sensor 4 supplies actual-value data which corresponds to the print result. This actual-value data is transferred to a comparator 6 via a circuit 5.

In addition, Figure 1 shows a device 7, that is separate from the printing machine 1, for supplying of the reference print. This contains a second optical sensor 8 - not represented in greater detail - that scans, in synchronism with the movement of the first sensor 4, a reference print 9 corresponding to the printed image of the printed product.

The reference print 9 is an error-free specimen.

By synchronous scanning, the first and second sensors 4,8 generate data flows that are transferred to the comparator 6 via the circuit 5 and a circuit 10. In real-time processing, the comparator 6 performs a comparison of the supplied data of the data flows. In this way, the actual-value data of the first sensor 4 is compared with the appropriate specifiedvalue data of the second sensor 8, whereby, in the case of sameness, or differences within a defined tolerance, the print result is declared to be faultless, and in the case of unacceptable differences between the actual-value data and the specified-value data, a rejection signal A is generated.

This rejection signal A then has the result that the corresponding printed product is rejected as a misprint.

The example of Fig. 2 differs from the example of Fig. 1 in that the device 7 for supplying of the reference print is positioned inside the printing machine 1. This has the advantage that installation of additional constructional units is not necessary. Of course, it is also possible to house the comparator 6 in the printing machine 1.

According to the invention, therefore, print quality is controlled through comparison of images, whereby, for the comparison of images, two synchronized sensors and original pictures are used. A printed, error-free specimen sheet is used in a reference-image storage (device 7 for supplying of the reference print). The comparison is performed in comparator 6 in real-time processing, i.e. no expensive electronic memory devices are necessary. A particularly advantageous aspect of the solution according to the invention is the especially quick access to the data records, whereby access to both data records (actual-value data and specified-value data) is of equal rapidity. To this is added the advantage that resolution and the image section can be quickly changed.

It will be appreciated that the invention has been described above by way of example only and that changes may be made without departing from the scope of the invention.

Claims

1. A device for controlling the print quality of printed products of a printing machine, with a first optical sensor, which supplies actual-value data, for scanning of a printed image of the printed product, and with a comparator which compares the actual-value data with defined specified-value data and generates a rejection signal in the case of unacceptable differences between actual-value date and specified-value data in which the specified-value data originates from a second optical sensor which scans, in synchronisation with the first optical sensor, a reference print corresponding to the printed image, and in which the comparator performs real-time processing of the data from the two sensors.

2. Device according to claim 1, in which the sensors perform line scanning and/or column scanning.

3. Device according to claim 1 or claim 2, in which the second sensor is located in a device for supplying a reference print which is separate from the printing machine.

4. Device according to claim 1 or claim 2, in which the second sensor is located in a device for supplying a reference print which is positioned inside the printing machine.

5. Device according to any one of the above claims, in which the first sensor is positioned inside the printing machine in such a way that it can scan the printed image of the printed product while it remains in the normal printconveyance cycle.

6. A method of controlling print-quality of printed products of a printing machine, comprising: scanning with a first optical sensor a printed image of the printed product; supplying actual-value data from the first optical sensor; scanning with a second optical sensor a reference print corresponding to the printed image, in synchronisation with the scanning of the printed image, to provide specified value data; comparing the actual-value data with the specified value data; performing real-time processing of the actual value data and the specified value data; and generating a rejection signal when there are unacceptable differences between the actual-value data and the specified-value data.

7. A method according to claim 6 for operation of a device according to any one of claims 1 to 5.

8. A method substantially as described with reference to figure 1 or figure 2 of the drawings.

9. A device substantially as described with reference to figure 1 or figure 2 of the drawings.