GB2114792A

GB2114792A - Recognising optical code markings

Info

Publication number: GB2114792A
Application number: GB08301249A
Authority: GB
Inventors: Dr Arthur Walter; Jurgen Erdmann; Gunter Fetzer; Felix Schorlin
Original assignee: Erwin Sick GmbH Optik Elektronik
Current assignee: Erwin Sick GmbH Optik Elektronik
Priority date: 1982-02-04
Filing date: 1983-01-18
Publication date: 1983-08-24
Also published as: GB2114792B; FR2520901B1; FR2520901A1; GB8301249D0; DE3203720C2; CH658138A5; DE3203720A1

Abstract

An apparatus for recognising optical coding marks applied to articles has two photoreceivers (11, 12) arranged one behind the other in the reading direction. The outputs of the photoreceivers (11, 12) are connected to a difference forming stage to compensate for mispositioning of the article. Further photoreceivers (30, 34) detect presence/absence of the article. Mirror 38 separates visible from infra-red. <IMAGE>

Description

SPECIFICATION Apparatus for recognising optical code markings applied to articles The invention relates to an apparatus for recognising optical code markings applied to articles and for their conversion into electronically processable signals, the apparatus comprising an optical illuminating device for imaging the code markings onto at least one photoreceiver and an electronic processing circuit connected to the photoreceiver, wherein relative movement takes place between the apparatus and the articles which results in the code markings being imaged after each other onto the photoreceiver.

In industry finished products, individual parts, lables, operating instructions, product descriptions etc. are nowadays provided with some form of printed code. Such codes make it possible, by way of example, to recognise when coded objects have become mixed up and thus allow such mixing up to be prevented. Furthermore, control, counting and registration operations can be initiated by means of such codes.

Apparatuses for recognising printed code, markings generally scan the code markings optically without contact and convert them into electrical signals which are capable of being processed. Furthermore an output signal, which specifies whether an object is actually in the reading station, is generally transmitted in order to activate the further processing of the pulse sequence generated by the code markings.

The problem however generally exists with such optical scanning systems that it is necessary to accurately guide the object bearing the code markings to be read. Larger tolerances in the scanning distance and/or the scanning angle impair the reliability of the reading or make correct reading impossible.

The object underlying the invention is now to provide an apparatus of the initially named kind in which larger tolerances in the scanning distance or scanning angle can be accepted without the reliability of the reading being impaired, or correct reading being made impossible.

In order to satisfy the object the invention provides that two photoreceivers are arranged one after the other in the reading direction at such a spacing that the code markings are imaged first on the one photoreceiver and then on the other photoreceiver, and that the outputs of the photoreceivers are connected to a difference forming stage, optionally via amplifiers.

A change of spacing of the code carrier and a relative change of the angle of the code carrier related to the scanning apparatus brings about a change in the photocurrent of the photoreceiver associated with the light reflected from the surface. In customary reading systems this results in the simulation of a jump in contrast due to a code bar.

By evaluating the photocurrents of the two photoreceivers of the invention and by using a difference forming stage, which is preferably constructed as a differential amplifier, the output signal of this stage does not show a change of level due to the initially mentioned disturbing factor "change of the scanning distance andangle" because the disturbing factor affects both photocurrents in the same way. If however a code marking passes beneath the reading system a change of level occurs in any event at the output of the difference forming stage. Thus a distinguishing feature is present for distinguishins between the change of photocurrent due to change of the scanning distance and/or angle and the change of photocurrent due to the reading of a code bar.

An advantageous constructidnal realisation of the proposal of the invention is so constructed that a comparator is connected to the difference forming stage with the comparator reacting when a predetermined value of the input signal is reached and with its output being applied to the control inputs to analog to digital converters. The signal inputs of the analog to digital converters are respectively connected to one of the photoreceivers so that, in the event that an output signal appears at the difference forming stage, the comparator initiates the digitising process in the analog to digital converters via the control inputs.

In this arrangement it should in particular be provided that the outputs of the analog to digital converter only allow resetting of the comparator via a return line when the outputs of the two analog to digital converters have transmitted pulses corresponding to the width of a code marking.

The resetting of the comparator is prevented by the feedback and indeed until both outputs of the analog to digital converter announce the end of the code marking.

The illuminated code markings which are to be read generate differential photocurrents in the two photoreceivers as a result of different reflective powers relative to the background. Differential photocurrents also arise by reason of a change of the distance and/or angle between the photoreceiver and the code markings. However, only the change in photocurrent generated by the code markings may lead to evaluation. This is ensured by the arrangement of the two photoreceivers one behind the other in the reading direction in accordance with the invention.

If however a code marking is first imaged on one of the photoreceivers an output signal is created at the output of the difference forming stage which initiates the digitising process via the comparator. The digitising process is allowed to continue by reason of the feedback of the digital signals to the comparator until pulses corresponding to the width of the code markings have been transmitted at the outputs of the analog to digital converters. It is thus ensured that pulses are only transmitted from the analog to digital converters when actual code markings are present and not simply when changes in the photocurrent are caused by changes of spacing and/or angle.

In order to specify the times at which the signals at the analog to digital converters are valid, or which pulse groups belong together, it is necessary to prepare a reading window. This can be realised either with an optical scanner or with a light barrier.

In accordance with a preferred embodiment it is accordingly provided that a third photoreceiver recognise the presence of a code marking carrier in the field of view of the apparatus and forms a reading window pulse therefrom. In this arrangement the third photoreceiver should be connected, if necessary via an amplifier, to an analog to digital coverter which transmits the reading window pulse. - The arrangement is preferably such that the third photoreceiver receives the infrared component of the received light. The arrangement should in particular be such that a colour dividing mirror separates the measuring light for the two first photoreceivers, which lies in the visible region, from the light which reaches the third photoreceiver.

It is important that the incandescent bulb used as a sender has a very wide excitation spectrum in the visible and infrared range. Information capable of evaluation is however primarily obtained, on reading the code markings, in the visible range because in the infrared range only low contrast is present between the various colours. For this reason only the visible component is evaluated during the measurement by splitting it off by means of the colour dividing mirror and preferably also by filtering the light reflected to the two first photoreceivers. The object illuminated by the optical illuminating device can be regarded as a good reflector in the infrared range. Even code markings which appear visually black reflect a considerable proportion of the incident light in the infrared range.The infrared light separated from the first two photoreceivers by the colour dividing mirror falls on the third photoreceiver. The output signal of the third photoreceiver is digitised by means of the said analog to digital converter and thus forms a reading window pulse.

As an alternative the third photoreceiver can however also be part of a light barrier which responds to the presence of the code marking in the field of view. The arrangement here can in particular be such that a reflector, which forms an autocollimation light barrier with the optical illuminating device and the other third photoreceiver, is arranged behind the code marking carrier.

It is particularly advantageous when two third photoreceivers are present which can be selectively connected to the analog to digital converter via a change-over switch. Thus, if the reading window pulse cannot be generated by means of the infrared sensor the autocollimation light barrier can be interrogated via the said change-over switch. For this purpose the reflector is positioned in the range of view of the light barrier and behind the object to be read. If no object is located in the beam path the third photoreceiver associated with the light barrier receives light reflected by the reflector. The beam path is however interrupted when an object is located in the reading position.

The output signal of the relevant third photoreceiver is then passed via the change-over switch to the analog to digital converter which in turn prepares the reading window pulse at its output.

As already mentioned the reading of the code markings produces a difference at the output of the difference forming stage as a result of the temporally displaced scanning. This difference is proportional to the difference of the reflective capability of the code marking carrier and of the code markings. As a minimal difference must occur in order to initiate the digitisation, the amplitude of the difference signal is suitable for judging the reliability of the reading. Large amplitudes show that the reading is particularly reliable.

By combining the two first code recognition photo receivers with the two, alternatively operable presence recognisi ng photoreceivers the reading head containing the entire arrangement can be operated self-sufficiently without additional aids. By processing the differential photocurrents of the two first photoreceivers it is possible to distinguish between a photocurrent change through a change in the scanning distance and a change of the photocurrent due to recognition of a code bar in order to initiate the reading. In accordance with the invention the difference of the photocurrents of the two first photoreceivers, which is proportional to the contrast, is used for judging the contrast. A distance gauge is preferably also provided in accordance with the invention.

The invention will now be described in the following with reference to the drawings which show: Fig. 1 a schematic illustration of the apparatus of the invention for optical recognition of printed code markings, Fig. 2 a schematic block circuit diagram of the electronic processing circuit of the apparatus of Fig. 1, and Fig. 3 a schematic representation of a preferred code used with the apparatus of the invention shown in relation to the schematically illustrated photoreceivers.

In Fig. 1 an incandescent lamp 41 and a condenser 42 form an optical illuminating device 40 which obliquely illuminates a code marking carrier 37 carrying code markings 29 with a somewhat converging light beam 43. In the region of the code marking carrier 37 the light beam 43 has an extent such that the code markings 29 are just completely illuminated.

Behind the code marking carrier 37 there is located a reflector 39 at right angles to the optical axis 44 of the optical illuminating device 40. The light of the incandescent lamp 41 is concentrated on the surface of the reflector 39, provided the code marking carrier 37 is not present.

The optical axis 45 of an optical receiving device 46 extends perpendicular to the code marking carrier 37 which is represented as being essentially flat. The optical receiving device consists of a lens 47 and a colour dividing mirror 38 which reflects out sideways the visible part of the received light and concentrates it, via a filter 48 which filters out the visible light, onto two photoreceivers 11, 12 arranged spaced apart alongside one another. In the embodiment of Fig.

1 it is assumed that the code marking carrier 37 is moved in a direction at right angles to the plane of the drawing so that the images of the code markings 29 developed by the lens 47 in the plane of the photoreceivers 11 and 1 2 fall one after the other on the photoreceivers 11 and 1 2. Indeed during movement of the code marking carrier 37 the image of the code mark 29 falls first of all on the photoreceiver 11 and then at a certain time later on the photoreceiver 1 2. At a specific time the image of the code marking 29 can thus be found either on the photoreceiver 11, or on the photoreceiver 12, or on both.

The photoreceivers 11, 12 are connected to the electronic processing circuit 50 which feeds the outputs 23, 24 and 49, which are described below in more detail with reference to Fig. 2, from the signals of the photoreceivers 11, 12.

The infrared light passing through the colour dividing mirror 38 falls on a further photoreceiver 30 which is likewise connected to the electronic processing circuit 50 and which feeds a further output 51 in the manner explained in relation to Fig. 2.

The reflector 39 cooperates with a further photoreceiver 34 via a small deflecting mirror 52 arranged on the optical axis 44 which reflects out sideways parts of the light reflected back on itself from the reflector 39 and deflects it to the photoreceiver 34. The small deflecting mirror 52 is arranged directly in front of the condenser 42.

The photoreceiver 34 is also connected to the electronic processing circuit 50 and feeds the output 51 in the manner described later with reference to Fig. 2. Finally the power supply of the electronic processing unit 50 also provides the current for the incandescent lamp 41 of the optical illuminating device 40 via a line 53.

As a result of the different reflection values of the code markings 29 and of the code marking carrier 37 differential light intensities are created in the reflected light. As a result of the movement of the code marking carrier light 37 at right angles to the plane of Fig. 1 the code markings 29 are imaged one after the other onto the photoreceivers 11 and 1 2. Processable photocurrent changes in the photoreceivers 11 and 1 2 are thus produced by the coloured code markings.

As a result of the presence of the photbreceiver 30 which accepts the component of the light in the infrared range transmitted by the colour dividing mirror 38 the photoreceiver 30 will always receive light when a code marking carrier 37 is present in the field of view of the arrangement and thus announce the presence of an article.

The autocollimation light barrier formed by the light beam 43, the reflector 39, the deflecting mirror 52 and the photoreceiver 34 likewise announces the presence of a code marking carrier 37 in the field of view because, on passage of the code marking carrier 37 past the apparatus, the beam path of the light barrier is interrupted and the photoreceiver no longer illuminated. Thus the photocurrent of this photoreceiver reduces, which signals the presence of an article. It is also important that an aperture stop 54 which only transmits the light reflected from the mirror 52 be arranged in front of the photoreceiver 34.

In Fig. 2 the photoreceivers 11, 12 are connected via amplifiers 21, 22 to the two inputs of a difference forming stage 1 3. In addition, the output signals of the amplifiers 21, 22 are passed to the inputs 1 9, 20 of analog to digital converters 17 and 1 8 respectively.

The output of the difference forming stage 13 leads in the first place to the output 49 of the electronic processing circuit 50 the important components of which are shown in Fig. 2.

The output of the difference forming stage 1 3 is also applied to a comparator 1 4 which feeds the control inputs 1 5, 16 of the two analog to digital converters 17,18.

Respective feedback lines lead to the resetting inputs 25, 26 of the comparator 14 from the outputs 23, 24 of the analog to digital converters 17, 18. If the photoreceivers 11, 12 receive the same quantity of light no signal is present at the output of the difference forming stage 1 3.

However, if a code marking 29 is first imaged on one of the photoreceivers, for example on the photoreceiver 11, an output signal is created at the output of the difference forming stage 13 which initiates the digitisation by the analog to digital converter 17, 1 8 via the comparator 14. By feeding the digital signals back to the resetting inputs 25, 26 of the comparator 14 the digitisation process is only allowed to take place until pulses 27 and 28 corresponding to the width of the code marking 29 have been transmitted from the outputs 23, 24, respectively.In this way it is ensured that pulses 27, 28 are only transmitted at the outputs 23, 24 of the analog to digital converter 17, 18 8 when actual code markings are present and not simply when photocurrent changes are caused by changes in spacing and/or angle.

Reading window pulses are generated by means of the photoreceiver 30, or 34, in order to specify the time at which the output signals at the outputs 23, 24 and 49 are valid. This can alternatively be realised with an optical sensor or with a light barrier.

The photoreceiver 30 which receives the infrared light from the object through the colour dividing mirror 38 is connected via an amplifier 31 and a change-over switch 36 to the analog to digital converter 33 at the output 51 of which the reading window pulse 32 appears.

If the reading window pulse 32 cannot be generated by means of the photoreceiver 30, the change-over switch 36 can be changed over into the other switch position in which it is connected with an amplifier 35 fed by the light barrier photoreceiver 34.

If the beam path through the code mark carrier 37 is interrupted, a corresponding reading window pulse 32 is generated in the analog to digital converter 33 and releases the readout.

The code reader recognises only grey value contrasts between the printed code and the code carrier. The coding itself, i.e. the individual bars can be applied in the most diverse colours, it being only the width of the code bars together with the intermediate gaps which contains the information.

Coloured code bars are used in practice because the printing on the lable, on the operating instmction or on the packaging is already in colour and one does not wish to have to carry out a special printing step to apply the code.

Furthermore, the code word can consist of the most diverse coloured bars. This is also the case when important information of a description is to be characterised by a special colour. If this colour is also an element of at least one code bar of the total code word it can simultaneously be controlled during the reading whether this important colour printing operation has been carried out.

The code word to be read consists, as seen in Fig. 3, of thick and thin bars with intermediate gaps. The width of the thick bar corresponds to three times the width of the thin bar. The gaps between two bars correspond to twice the width of the thin bar. The dimensions of the two photoreceivers 11, 12 are so selected, taking account of the magnification by the optical system, that they are in each case 0.4 mm wide and the centres of the two receivers are 0.5 mm apart. This signifies that both receivers see the background between each coding bar. A thick coding bar will fall on both receivers. The colours of the individual code bars do not bear any relationship to the information of the code word.

As a gap of 1 mm width is present between two differently coloured code bars the condition will never occur that both photoreceivers see different colours.

Finally, each individual photoreceiver operates on the associated electronic processing circuit in the scanning head so that two contrast mark scanners with electronics are present. If the photocurrents of the two photoreceivers are identical, and if the output voltage of the difference forming stage 1 3 thus goes to zero, the associated analog to digital converters 1 7 and 1 8 are switched out. If a difference in the photocurrents of the two photoreceivers occurs, which is synonoymous with the entry of an edge of a contrast mark, the converters 1 7 and 18 are activated and now evaluate the voltage jump of the associated photoreceiver related to the photocurrent value of the individual receiver immediately before activation. The resetting of the signal permitting processing is forbidden via a feedback line for the duration of the jump in contrast that is present.

Claims

1. Apparatus for recognising optical code markings applied to articles and for their conversion into electronically processable signals, the apparatus comprising an optical illuminating device for the article, an optical receiving device for imaging the code markings onto at least one photo receiver and an electronic processing circuit connected to the photoreceiver, wherein relative movement takes place between the apparatus and the articles which results in the code markings being imaged after each other onto the photoreceiver, characterised in that, two photoreceivers (11, 1 2) are arranged one after the other in the reading direction at such a spacing that the code markings (29) are imaged first on the one photoreceiver (11) and then on the other photoreceiver (1 2), and in that the outputs of the photoreceivers (11, 12) are connected to a difference forming stage (13), optionally via amplifiers (21,22).

2. Apparatus in accordance with claim 1, characterised in that a comparator (14) is connected to the difference forming stage, the comparator reacting when a predetermined value of the input signal is reached and its output being applied to the control inputs (15, 16) of two analog-digital converters (17, 18!, the signal inputs 9,20) of which are respectively connected to one of the photoreceivers (11, 12 respectively), wherein, in the event that an output signal appears at the difference forming stage (13), the comparator (14) initiates the digitising process in the analog to digital converters (17, 18) via the control inputs (1 5, 1 6).

3. Apparatus in accordance with claim 2 and characterised in that the outputs of the analog to digital conveters (1 7, 18) only allow the resetting of the comparator via a return line when the outputs of the two analog to digital converters (17, 18) have transmitted pulses corresponding to the width of a code marking.

4. Apparatus in accordance with one of the preceding claims and characterised in that a third photoreceiver (30, 34) recognises the presence of a code marking carrier (37) in the field of view of the apparatus and forms a window reading pulse (32) therefrom.

5. Apparatus in accordance with claim 4 and characterised in that the third photoreceiver (30, 34) is connected, if necessary via an amplifier (31, 35), to an analog to digital converter (33) which transmits the window reading pulse (32).

6. Apparatus in accordance with claim 4 or claim 5 and characterised in that the third photoreceiver (30) receives the infrared component of the received light.

7. Apparatus in accordance with claim 4 or claim 5 and characterised in that the third photoreceiver (34) is part of a light barrier which responds to the presence of the code marking carrier (37) in the field of view.

8. Apparatus in accordance with claim 6 and 7 and characterised in that two third photoreceivers (30, 34) are present which can be selectively connected to the analog to digital converter (33) by a change-over switch (36).

9. Apparatus in accordance with claim 6 or claim 8 and characterised in that a colour dividing mirror (38) separates the measurement light for the two first photoreceivers (11, 12) which lies in the visible range from the light which reaches a third photoreceiver (30).

10. Apparatus in accordance with claim 7 or claim 8 and characterised in that a reflector (39), which forms an autocollimation light barrier with the optical illuminating device (40) and the other third photoreceiver (34), is arranged behind the code marking carrier (37).

11. Apparatus substantially as herein described and illustrated with reference to the accompanying drawings.