GB2065871A

GB2065871A - Web register control

Info

Publication number: GB2065871A
Application number: GB7943345A
Authority: GB
Original assignee: Crosfield Electronics Ltd
Current assignee: Crosfield Electronics Ltd
Priority date: 1979-12-17
Filing date: 1979-12-17
Publication date: 1981-07-01

Abstract

Apparatus for detecting errors in longitudinal register of register marks (1, 2, etc.) printed with a given spacing (d) along a web, comprises a single detecting means responsive to the passage of the marks as the web moves, to provide corresponding mark signals, and an error computing circuit responsive to the interval between the mark signals and to a web speed signal to compare the longitudinal spacing of a pair of marks with the spacing for correct registration and to provide an output signal indicative of the longitudinal register error. The apparatus may also detect errors in lateral register, using marks which vary in thickness along their width, by responding to the thickness of the marks and hence to their lateral position. This apparatus allows smaller mark separation, and a smaller and simpler scanning head, than previous apparatus using a two-channel scanning head. It can be programmed to detect register error between any two sets of register marks and to compensate for changes in mark separation due to web stretching. <IMAGE>

Description

SPECIFICATION Web register control This invention relates to an apparatus for detecting errors in longitudinal and lateral register of operations performed on a web at successive stations.

Previous methods for measuring the register error between any two sets of register marks on the web have used a two-channel scanning head consisting of two light sources and two photodetectors, with a separation equal to the desired separation between each pair of register marks which are to be compared, as shown in Fig. 1. Referring now to Fig. 2, the output from each detector shows two distinct peaks corresponding to the register marks. If the marks are correctly registered longitudinally, then detector 1 should obtain its second peak and detector 2 its first peak simultaneously. Any deviation from this condition is indicative of longitudinal register error. Lateral register error may be detected by the same detectors, by providing register marks of a suitable shape.

Figure 3 shows two typical examples. The distances S1 and S2 are proportional to the width of the first and second peaks in either detector output; by similar triangles, S1 and S2 are also proportional to the lateral distance from one side of the mark to the detector, and hence the difference between Si and S2 is proportional to the lateral distance between the two register marks, i.e. to the lateral register error.

The principal disadvantages of this apparatus are as follows. The scanning head must house two detectors with the correct separation, and is therefore costly and bulky. The distance between the pairs of register marks must always be the same, for a given set of scanning heads so the scanning heads are restricted to error measurement between one set of marks and only one other. Furthermore, the distance between the pairs of register marks cannot be iess than the minimum practical distance between two detectors, which is generally around 20 mm, so a substantial web distance is necessary for mark laydown. Because the distance between marks is fixed, webs which shrink after printing by varying degrees cannot be registered by this method, for.

example when printing on extensible films.

It is the object of this invention to provide an improved apparatus for detecting both longitudinal and lateral register errors.

The invention consists in register control apparatus for use with a web on which register marks corresponding to operations performed on the web are printed with a given spacing in the direction of web travel for correct registration of the operations, the apparatus comprising a single detecting means which is responsive to the passage of the register marks on the web to provide corresponding register mark signals, means for providing a web speed signal varying with the speed of the web, and an error computing circuit responsive to the interval between the register mark signals and to the web speed signal to compare the relative longitudinal position of a pair of register marks with the relative position for correct registration of the said operations performed on the web and to provide an output signal indicative of the register error in the longitudinal direction.A preferred embodiment of the invention will now be described with reference to Figs. 4 to 9 of the accompanying drawings.

Figure 4 shows the preferable shape of the web marks, which are similar to those shown in Fig. 3.

The print detector responds to each mark and sends signals similar to those in Fig. 5 to a control circuit, shown schematically in Fig. 7.

Figure 4 shows 4 register marks representative of four different operations on the web, and this series would be repeated along the length of the web. The register error between two such operations is obtained by comparing the positions of a pair of the register marks, e.g. marks 1 and 2.

Longitudinal error is obtained in the control circuit by measuring the interval (t) between the instants that the leading edges of a pair of register marks pass the detector, and by simultaneously measuring the web speed (v). The web speed is obtained from the rotational speed of the print cylinder, as measured by a digital generator B, and the cylinder circumference, which is given as a manual input to the control circuit by the operator.

The distance d between the pair of marks is then computed as vt, and subtracted from the correct distance to obtain the error.

The lateral register error is obtained in the detector circuit by measuring the interval between the leading and trailing edges of each mark passing the detector, and computing the difference between the values of this interval (t, and t2) for a pair of marks, as shown in Fig. 6. The interval multiplied by v is equal to the width of the mark as it passes the detector, so the difference between the intervals for a pair of marks multiplied by v is equal to the difference in width between the marks, which, by similar triangles, is proportional to the lateral register error.

The detector circuit obtains the longitudinal register error and the mark thickness difference which is proportional to the lateral error, and provides signals indicative of these quantities as outputs, for example to a mechanism for correcting the errors.

The detector circuit will now be described in greater detail, with reference to Figs. 7. 8 and 9.

The web scanning head A provides a print signal 1. In order to distinguish between peaks due to printed material and those due to web marks, a gating signal generator C provides signals 2 and 3 for gating the print signal at times when the register marks should be close to the detector. Signal 3 gates the print signal for mark 1 , the first of the pair of marks, and signal 2 gates the print signal for mark 2, the second of the pair.

Gating circuits E and F select only the relevant peaks from the print signal, and provide signals 5 and 4 respectively. E and F also have the function of compensating for unequal amplitudes of the peaks in the print signal, by providing an output signal of consistent amplitude (see Fig. 6).

A counter G is started by signal 4 and stopped by signal 5, thus measuring the interval t between the two marks passing the detector. The counter G also receives web speed information from a circuit D, to which is input the cylinder circumference and the cylinder rotational speed from the digital generator B. The counter G computes the mark separation and sends a signal 8 to a circuit H which compares this mark separation with a predetermined set mark separation, input as 9, and computes the difference between these separations, providing a signal 10 indicative of the longitudinal register error.

Signals 4 and 5 are also sent to mark width counters J and K respectively. Each counter is responsive to the width of the peak in each signal, i.e. to the interval between the instants that the leading and trailing edges of the mark pass the detector. Counter J counts t2, and counter K counts t. Counters J and K also receive signal 7 indicating the web speed, and output signals 11 and 1 2 indicating the mark thicknesses of marks 2 and 1 respectively. A circuit L receives signals 11 and 12 and computes the difference between the mark thicknesses, which, by similar triangles, is proportional to the lateral register error, and provides an output signal 13 indicative of the lateral register error.

Apparatus embodying this invention allows for more flexibility in its use than other apparatus.

Firstly, the mark separation may be made smaller, even to a size comparable with the width of the photodetector in the scanning head, and this requires less web distance for mark laydown.

Secondly, the scanning head is smaller and its design simpler. Thirdly, the register may be measured between any two sets of register marks printed in line, because the apparatus may be programmed (by signal 9) to any mark separation. This is an advantage when each colour is registered not always with the previously printed colour but with the first printed colour, requiring scanning for different mark separations after each print roller, as for example when corrections are made via differential gears.

Claims

1. Register control apparatus for use with a web on which register marks corresponding to operations performed on the web are printed with a given spacing in the direction of web travel for correct registration of the operations, the apparatus comprising a single detecting means which is responsive to the passage of the register marks on the web to provide corresponding register mark signals, means for providing a web speed signal varying with the speed of the web, and an error computing circuit responsive to the interval between the register mark signals and to the web speed signal to compare the relative longitudinal position of a pair of register marks with the relative position for correct registration of the said operations performed on the web and to provide an output signal indicative of the register error in the longitudinal direction.

2. An apparatus according to claim 1, wherein the device which provides a signal varying with the speed of the web is a digital generator associated with a web roller.

3. An apparatus according to claim 1 or 2, wherein the single detecting means comprises a lamp for illuminating a spot on the web and a single photocell detector responsive to the reflected brightness of the spot for providing the register mark signals.

4. An apparatus according to claim 1, 2 or 3, wherein the error computing circuit comprises a gating signal generator, responsive to the web speed signal, which supplies gating signals to a pair of gating circuits which are also responsive to the register mark signals, and which gate the register mark signals with the gating signal to select first and second register mark signals indicative of the detection of the first and second of the said pair of register marks respectively.

5. An apparatus according to claim 4 as appendant to claim 2, wherein the error computing circuit further comprises a means for receiving as input a signal representing a dimension of the web roller, which it uses with the signal from the digital generator to compute the web speed.

6. An apparatus according to claim 4 or 5, wherein the error computing circuit further comprises a counter circuit responsive to the first and second register mark signals, which provides a length signal varying with the distance between first and second register marks; and a circuit responsive to the length signal which compares this signal with a signal representing a set mark separation to provide an output signal varying with the longitudinal register error.

7. An apparatus according to claim 4, 5 or 6, for use with a web on which are printed register marks in the form of filled-in right-angled triangles with the shortest sides aligned parallel and perpendicular to the web edges, wherein the error computing circuit further comprises first and second mark width counters responsive to the first and second register mark signals respectively which provide mark thickness signals varying with the longitudinal thickness of each register mark as it passes the detector; and a circuit responsive to the mark thickness signals which computes the difference in thickness and generates an output signal varying with the lateral register error.

8. An apparatus according to claim 4, 5 or 6, for use with a web on which are printed register marks in the form of a pair of skew lines, wherein each register mark signal consists of two peaks, and wherein the error computing circuit further comprises: first and second mark width counters responsive to the peaks in the first and second register mark signals respectively, each of which provides a mark thickness signal varying with the longitudinal separation of the pair of skew lines at the point at which they pass the detector, this separation being dependent on the lateral position of the register mark; and a circuit responsive to the mark thickness signals which generates an output signal varying with the lateral register error.

9. An apparatus according to claim 6, 7 or 8, wherein the error computing circuit is responsive to the web speed signal, and in which the register error is computed by multiplying the web speed by the relevant time difference, and the output signal is directly indicative of the register error.

10. Register control apparatus substantially as described herein with reference to Figs. 4 to 9 of the accompanying drawings.