FR2555337A1 - Guided device for optically reading bar codes - Google Patents

Abstract

The present invention relates to a guide device for optically reading a bar code. The reading device includes, between the transmitter-receiver pair 21, 22 a diaphragm 28 whose dimensions optimise the resolution and the sensitivity of the reader.

Description

OPTICAL READING GUIDE DEVICE
FROM BAR CODES
The present invention relates to a guided optical barcode reading device.

 A bar code consists of a sequence of dark lines placed on a support separated by light spaces. The dark lines are absorbing optical radiation while the light spaces are reflective. An optical reading of bar codes by guided system can be carried out either by scrolling the bar code support or by scrolling the optical reading head. In both cases the relative movement of the head relative to the support is always directed along a perpendicular to the direction of the dark lines constituting the bar code.

 It is known to read only one bar at a time, in particular by using a double focus on transmission and on reception.

Such a solution, although allowing the information arranged in the form of a bar code to be properly reconstructed, is however an expensive solution.

 The present invention makes it possible to solve the problem using a very simple means and at a cost capable of allowing the mass production of an optical barcode reader.

 In an alternative embodiment, the invention proposes a simple means for carrying out an optimal reading of the bar code which spares at the same time the sensitivity, the resolution and the speed of reading.

Indeed, the invention makes it possible to carry out an optical reading at indifferent scrolling speeds. That is to say, the system is not forced to use only fast scrolling speeds greater than 7 cm / second as is the case in the prior art.

 Indeed, the present invention relates to a guided device for optical reading of bar codes. A device according to the invention comprises a diaphragm disposed between the reading device and the bar code support.

Other characteristics and advantages of the invention will appear in the description which follows and in the appended figures which are:
FIG. 1, a diagram of an optical reading device,
FIG. 2, a diagram of an optical reading device according to the invention,
FIGS. 3, 4 and 5, diagrams of the output signal of the reading device according to the invention in different configurations,
FIG. 6, a diagram of an optical reading according to the prior art,
FIG. 7, a diagram of an optical reading according to the invention,
- Figure 8, a diagram of an optical reading according to the invention.

 In Figure 1 there is shown the lower part 1 of a guided device for reading a support 2 of bar codes placed on its plate 12. The reading device comprises a receiver 4 of the optical flux retransmitted by the bar code illuminated by the transmitter 5. Note that if no special precautions are taken, the bars 8-11 are all arranged in field 6 of the transmitter E.

Similarly, the field of reflection is not fully picked up by the receiver 4. Such a device does not allow a reading of a tight bar code, an essential condition for a relative density of the information deposited in the form of bar code. on the support 2.

 According to the prior art, there is in the chamber 3 of the reading device 1 a means of optical focusing both on transmission and on reception. Such devices are expensive because they must be precise, the thickness of the lines having to be around one third of a millimeter.

 In Figure 2 there is shown a guided device according to the invention. The read head 20 comprises a transmitter 21 and a receiver 22. After the chamber 23 is interposed a diaphragm 28 between the radiation emitted in the beam 24 and the support 26 of the bar codes. In the window of width d of the diaphragm, we notice the presence of a bar 29 which absorbs or reflects a beam 25 of light on the receiver 22. The diaphragm is of a thickness K. The depth of focusing is designated by H point of the device 20 without diaphragm. This last datum is characteristic of the optical read head. The relative movements of the head 20 and of the support 26 are estimated from the plate 27.

 FIG. 6 shows a particular example of an embodiment according to the prior art and an embodiment according to the invention in FIG. 7. In FIG. 6 a double barcode support 30 is shown distributed over two tracks 31 and 32. The two tracks are arranged symmetrically with respect to the axis X of the scrolling of the support. Each track comprises a series of rectangular bars like bars 33 and 34. The spot 36 represents the impact of the light flux emitted by the focused transmitter of the optical pickup.

Note that in the example drawn, in order to read the two tracks 31 and 32 simultaneously, we are forced to have four rectangular bars in the field. In this hypothesis the reader cannot function. It will then be necessary to use bar codes whose information density, that is to say the number of bars per unit of length, is reduced.

 In Figure 7, according to the invention, there is a rectangular diaphragm elongated so that it encompasses the two tracks simultaneously. The diaphragm of width d projects a spot 41 perpendicular to the axis X of the scrolling. During the scrolling, we note that the rectangular bar 42 will disappear from the field while the rectangular bar 43 will enter the spot 41. In such a device, the transmitter-receiver pair 21, 22 of FIG. 2 must be doubled, a pair transceiver being associated with the high track and another pair transceiver being associated with the low track.

 In FIG. 8 another embodiment is shown according to the invention, the recording medium 50 carrying only along the scrolling axis X a single succession of information represented for example by dark dots whose the distribution constitutes the information recorded. A spot 51 of circular shape frames a point 52 giving a synchronization signal, for example for an electronic circuit for decoding the recorded information. In general, the diaphragm is geometrically identical to that of the element (line, bar or point) of the code read.

 In FIGS. 3 to 5 diagrams have been shown which will make it possible to explain a choice according to the invention of the characteristics of the diaphragm.

Three parameters are to be taken into account in this exemplary embodiment. For a given optoelectronics it is a question of achieving the best compromise between sensitivity, resolution and reading speed. This compromise is reached during the tests summarized in Figures 3 to 5 when the following two conditions are met:
- the modulation rate defined, from the height of the signal obtained at the device output, must be as high as possible;
- the zones of minimum and maximum intensities seen by the photodetector must form steps of equal dimensions. These conditions are met for a diaphragm width d such that:
d = e2 + a where e is the width of the bars and the minimum spacing between two bars. For this width d of diaphragm, the widths of steps Ps and pi are equal and such that:

 As examples of operation, different configurations of barcode and diaphragms with scrolling at constant speed are studied.

In all cases, it is noted that when the first bar enters the field of the photodetector, the output signal progressively passes from a maximum to a minimum. We obtain a lower bearing of width Pi-
When the first bar leaves the field, the signal gradually rises until it reaches an upper level of width.

In FIG. 3, three bars of a bar code are shown, the bars of which are of width e and of spacing a such that:
e = a2 and a diaphragm of three curves (E, x) giving the output signal from the photodetector as a function of its displacement x.

Each curve is obtained for different widths d of diaphragms.

It can be seen that the two conditions imposed are satisfied only the second curve of the diaphragm. Indeed, although the modulation rate is the same for the three cases considered, "level equality is only achieved in the second case. We find, for this case:

In FIG. 4, a second bar code is shown, the width of bars e being equal to the minimum spacing a:
e = a
Here, too, we can see that the two imposed conditions are only satisfied by the second curve of the diagram. Indeed, although the equality of the stages is obtained for the three cases considered, the highest modulation rate corresponds to the second case. We find again, for this case:

In FIG. 5, a third bar code is shown, the bar width of which is twice the smallest spacing a, that is:
e = 2a
Again, it can be seen that if the modulation rate does not vary for the three cases considered, the equality of the stages is only obtained for the second case.

Here again, this case corresponds to:

 In order to work without risk of saturation of the photodetector, the internal face of the diaphragm facing the emitter E is absorbing the radiation it emits.

Claims (6)

 I. Guided optical barcode kstre device, characterized in that it comprises a diaphragm (28) disposed between the reading device and the barcode support.  2. Device according to claim 1, the code being of the double bar type, characterized in that the diaphragm (28) simultaneously covers the two tracks of the code.  3. Device according to claim 1, characterized in that the diaphragm (28) is of the same shape as the elements of the code.  4. Device according to one of the preceding claims, the code bars being rectangular, characterized in that the diaphragm (28) is a rectangle.  5. Device according to one of the preceding claims, characterized in that the diaphragm (28) is of width (d), in the direction of travel, equal to half the sum of the width of the code element and of the minimum space between two elements.  6. Device according to one of the preceding claims, characterized in that the face of the diaphragm (28) facing the emitter (21) is absorbing the radiation <24) emitted.