GB2217833A

GB2217833A - Registration mark detection system

Info

Publication number: GB2217833A
Application number: GB8810343A
Authority: GB
Inventors: Peter Cox-Smith
Original assignee: Cranfield Institute of Technology
Current assignee: Cranfield Institute of Technology
Priority date: 1988-04-30
Filing date: 1988-04-30
Publication date: 1989-11-01
Also published as: GB8810343D0

Abstract

The system is for detecting marks provided on a moving web for positional control of a cutting or like process to be carried out on the web. It comprises an optical sensor 18 producing output signals when marks are sensed as the web moves past the sensor, a controlled-pulse generator 22 activated by the sensor output and for controlling the process machine and a signal conditioner which includes an a.c. coupling, eg differentiator 28, for supplying the controlled-pulse generator only with sensor output signals representative of high speed changes in said sensor output. <IMAGE>

Description

Title Detector System for Registration Marks Field of the invention This invention relates to a system for detecting marks provided on a moving web, typically of paper or card, for the positional control of a process, such as printing and/or cutting, to be carried out on the web. Such marks are referred to herein as registration marks.

Background to the invention In the paper and card industry, in particular, it is conventional to use an optical sensor to detect registration marks as aforesaid. The moving web on which the marks are provided may be a continuous web or a series of cut sheets, and the mark is usually but not essentially dark on a light background. The sensor produces a pulse when a mark is detected. A comparator is used to detect this pulse and thereby drive an output device handling the power interface to the machine in which the paper or card is being processed. The output pulse is thereby utilised to synchronise the machine processes with the movement of the web.

The conventional detecting system has several disadvantageous characteristics. First, the comparator threshold has to be set for a particular combination of background and mark colours. Secondly, the system is liable to respond to gradual change in background colour causing the comparator threshold to be crossed. Thirdly, the system can only be satisfactorily employed when the web is moving relatively slowly, partly because it is usually necessary to set the comparator threshold when the web is stationary, and adjust the set level to allow for web movement.

It is an object of this invention to provide an improved registration mark detector system which at least partly overcomes the disadvantages of the conventional system.

The invention According to the invention, there is provided a registration mark detector system comprising an optical sensor, a controlled-pulse generator for producing output pulses to be used for machine control and, between the sensor and the controlled-pulse generator, an a.c.

coupling for supplying the controlled-pulse generator only with signals representative of high speed changes in the sensor output.

Thus, in accordance with the invention, the detector system detects the speed at which a change in sensor output occurs due to passage of a registration mark, instead of the magnitude of this change, as in a conventional detecting system.

It will thus be appreciated that the detecting system in accordance with the invention is operational only when the web is moving. However, the production of output pulses to be used for control is affected by different background/mark colour combinations, is not affected by gradual background colour changes, and is especially suitable for detection of marks on a web moving at relatively high speed.

When a conventional optical sensor having a light transmitter and receiver for light reflected from the web is employed, this first produces a pulse of conventional form. A differentiating circuit, conveniently in the form of a capacitor/resistor coupling, is then employed to produce spikes from the leading and trailing edges of the sensor output pulse, preferably-after amplification of the latter. The controlled pulse generator produces pulses of controlled shape from the aforesaid spikes, again preferably after amplification of the latter. The amplifier between the differentiating circuit and the controlled pulse generator may be of a form to suppress either the leading edge or trailing edge spikes, or of a form to respond to both positive and negative spikes, thereby to enable wide bar registration marks to be used for dual machine control purposes.

The system in accordance with the invention requires for setting up only the setting of an appropriate frequency threshold for the a.c. coupling, having due regard to the minimum speed of the moving web when the machine is in use and the nature of the registration marks, which may be narrow lines, wide bars, and of any type which modulates the light reflected by the web, for example as due to a colour change or a surface groove or rib. Colour changes may require use of a monochromatic or filtered light source in the sensor.

Description of embodiment The detector system in accordance with the invention is exemplified in the following description, making reference to the accompanying drawings, in which: Figure 1 is a diagrammatic representation of a machine for processing a moving web; Figure 2 is a block diagram of a signal handling circuit in accordance with the invention; and Figures 3A to 3E are illustrative of the signals produced in the signal handling circuit in the case of registration marks existent against different background levels.

Referring to Figure 1, a continuous web 10 moving over rollers 12 bears registration marks which typically may be line registration marks 14 or colour block registration marks 16. An optical sensor 18 senses the marks 14, 16 and via a signal conditioning means 20 drives a controlled-pulse generator 22 which provides synchronisation pulses of controlled shape and level to the power interface of a machine actuator 24 which supervises a process such as printing or cutting to be effected at preselected positions on the moving web.

Referring to Figure 2, the signal condition means comprises amplifier 26, capacitor/resistor a.c. coupling 28 which acts as a differentiating circuit, and amplifier 30.

Referring now to the left-hand side of Figure 3, Figure 3A shows a registration mark 32 on a light background 34.

Figures 3B to 3E show the signals existent at points B, C, D and E in the block circuit diagram of Figure 2. A similar set of signals is shown on the right-hand side of Figure 3 for a registration mark 36 on a darker and less contrasting background 38. These two situations could not be handled by a conventional detector system without re-setting threshold levels. However, from Figure 3 it can be seen that the circuit of Figure 2 handles both situations with equal facility and without any resetting.

Various modifications of the above-described and illustrated arrangement are possible within the scope of the invention as hereinbefore described. For example, it will be appreciated that the registration marks may take the form of a bar code, as the above-described sensor and signal processing circuit may readily be adapted to read the code and provide a series of data or information signals to the machine controller.

Claims

1. A registration mark detector system for detecting marks provided on a moving web for positional control of a process to be carried out on the web, comprising an optical sensor for sensing the registration marks and providing output signals responsively to the sensing, a controlled-pulse generator for producing output pulses to be used for machine control responsively to the sensor output signals and, between the sensor and the controlledpulse generator, an a.c. coupling for supplying the controlled-pulse generator only with signals representative of high speed changes in the sensor output.

2. A system according to claim 1, wherein the optical sensor has a light transmitter and a receiver for light reflected from the web.

3. A system according to claim 2, having a differentiating circuit for producing spikes from the leading and trailing edges of each sensor output pulse.

4. A system according to claim 3, wherein the differentiating circuit is a capacitor/resistor coupling.

5. A system according to claim 3 or claim 4, wherein the sensor output is amplified prior to differentiation.

6. A system according to claim 3 or claim 4 or claim 5, wherein the controlled-pulse generator produces pulses of controlled shape from the spikes.

7. A system according to claim 6, wherein the spikes are amplified before being fed to the controlled-pulse generator.

8. A system according to claim 7, wherein the amplifier between the differentiating circuit and the controlledpulse generator is of a form to suppress either the leading edge or trailing edge spikes.

9. A system according to claim 7, wherein the amplifier between the differentiating circuit and the controlledpulse generator is of a form to respond to both positive and negative spikes, thereby to enable wide bar registration marks to be used for dual machine control purposes.

10. A system according to any preceding claim, having a monochromatic or filtered light source in the sensor for detecting registration marks represented by colour changes.

11. A registration mark detector system substantially as hereinbefore described with reference to the accompanying drawings.