GB2167221A - Bar code readers - Google Patents

Abstract

A bar code reader operates in real time and has a vidicon camera unit 10 which operates with an interlace ratio which falls in the range of 3:1 to 9:1. The reader uses analogue signal conditioning circuitry to minimise the residual effects of camera lag and to maximise the electronic signals obtained from the camera when a code passes at speed in front of it. The resultant restored signal is then converted 12, from an analogue signal into a digital signal and clocked into a logic circuit 14, 16, 18, that decodes the signal in real time with reference to parameters defined for the code type being used. All the decoding takes place within the time taken to scan once across the field of view. However, a number of scans are taken to check the results before a number is validated and the code number is passed to output circuitry. <IMAGE>

Description

SPECIFICATION Improvements in bar code readers The present invention relates to bar code readers of the type which are mounted at a fixed location and are intended to read bar codes on goods as the latter move past.

Known bar code readers of this type usually use a laser tube as the input transducer.

These suffer from the disadvantages in practice of (a) requiring the bar codes to be printed accurately to a high tolerance, (b) neing unable to read bar codes printed at the red end of the light spectrum, (c) being of relatively slow operation so as to be incapable of operating with fast moving goods, and (d) requiring close proximity to the goods, so rendering them unsuitable in many applications.

A camera based reader is also known but this has the disadvantages that (a) it can only read its own specified code format, (b) it can only read "ladder" type codes-i.e. a stack of horizontal bars, and (c) it is bulky and includes a 19" rack mounted unit situated remotely from the camera.

It is an object of the present invention to provide a bar code reader wherein the disadvantages associated with the aforegoing known devices are mitigated and which can (a) read a wide variety of code formats,(b) read quickly and reliably in both the "ladder" code orientation and also the "picket fence" orientation, i.e. one using a plurality of laterally spaced, vertical lines, (c) read multicolour codes, including red, and (d) operate in real time to avoid the use of complex and expensive information storage devices.

In accordance with the present invention, there is provided a bar code reader which operates in real time and which includes a vidicon camera unit adapted to operate with an interlace ratio which falls in the range 3:1 to 9:1.

Such a reader has the advantage of not requiring the use of large and expensive frame stores and onboard computers.

The reader uses a non-standard scanning coil arrangement and drive electronics for producing a new relationship between line and field frequencies from that used in standard T.V. cameras. The actual relationship between the line and field frequency is dependent upon the type and size of the code being used.

In addition, the present system uses analogue signal conditioning circuitry on the output from the transudcer to minimise the residual effects of camera lag and to maximise the electronic signals obtained from the transducer when a code passes at speed in front of it.

The resultant restored signal is then converted from an analogue signal into a digital signal and clocked into a logic circuit that decodes the signal in real time with reference to parameters defined for the code type being used. All the decoding takes place within the time taken to scan once across the field of view. However, a number of scans are taken to check the results before a number is validated and the code number is then passed to the output circuitry.

At the output stage the decoded number can then be recoded into ASC II standard coding for transmission over a standard computer interface.

The invention is described further hereinafter, by way of example only, with reference to the accompanying drawings, in which: Fig. 1 is a block schematic diagram of one embodiment of a bar code reader in accordance with the present invention; Figs. 2 and 3 are diagrammatic representations for use in explaining the invention and referring to the prior art and an embodiment in accordance with the invention, respectively; Fig. 4 is a schematic diagram of one embodiment of an interlace generator for a reader in accordance with this invention; and Fig. 5 shows the output waveform of a 4:1 interlace generator such as that illustrated in Fig. 4.

The illustrated bar code reader is based on a vidicon camera which is arranged to operate with real time data processing to analyse, recognise and output the alpha-numeric data presented within the viewing area, whether moving or stationary.

Within the processing operation, checks are made for all of the parameters associated with the specific code type, the start and stop digits, and bar and space widths, together with the number of bars and spaces and a running total of expected characters.

The code, once recognised, is then stored and compared with further "reads" in order to validate the original "read". Upon receipt of the correct number of valid "reads", the code is deemed to be "true" and is made available at an output data port.

Fig. 1 shows the fill system in block form.

A specially designed vidicon camera unit 10 (described in detail hereinafter) provides at its output a video monochrome signal which has analogue processing applied to it by means of a dynamic analogue slicer unit 12 which maintains an average level to which the analogue video signal is compared to produce a single digital output.

This digital signal is applied to bar and space discriminators 14, 16. A bar is arranged always to correspend to a specific logic level and a space to its complement so that the two outputs can never be confused.

The bar and space information, in quantised form, is fed to a FROM decoder 18 which interprets this quantised data into the true binary representation of the associated code alpha-numeric symbol.

A series of checks is performed on the quantised data from the bar/space discriminator 14, 16 and the FROM decoder 18 by units 20, 22, 24, 26 to validate the start and stop bits and the quantity of bars and spaces in each character.

The start and stop bit recognition 20 to 26 also performs the function of determining the "read direction", be it forward or reverse, and this information bit is used to control the manner in which the data is read into the FROM decoder output buffer 28.

A verification of the expected number of wide and narrow bars is made to check the absolute authenticity of the individual character set within the total length of the bar code.

If any one of the aforegoing checks proves false then the "read" is eliminated from any further processing and the unit awaits further incoming data.

When the verifications are all "true", a store update signal is outputted which controls an output store 30.

If without a false "read" enough correct recognitions are made, then the code is deemed to be correct and a "code true" output is given.

If, however, during a sequence of reads a false code is discovered then the system is reset and awaits further incoming data.

The vidicon camera unit is now described in more detail.

It is well known that the vidicon camera tube exhibits a lag characteristic when a subject is moved in front of it. This causes blurring of the video signal and results in a lack of high resolution definition which is normally an inherent feature of conventional vidicon devices.

The aforedescribed blurring occurs when the subject matter is placed parallel with the line scan. In practice, the printing of the bar code to the subject goods is easily performed horizontally and this produces a code whose direction of "read" is parallel with the line scan of the camera system.

For the present purposes, the vidicon unit was chosen for its longevity, robustness and its initial low cost and availability. However, the lag characteristic inherent in any standard television signal format has previously made recognition of codes moving at speeds greater than five metres per minute, in the parallel scan mode, virtually impossible.

The presently proposed camera unit has overcome the lag associated problem by scanning the surface of the vidicon target in a different manner as described hereinafter.

All present accepted T.V. standards or derivitives scan the camera face from left to right in a few microseconds and, within a corresponding number of milliseconds, scan from top to bottom, twice, giving two fields of a few hundred lines constituting a t.v. frame (see Fig.2).

This technique is known as interlacing and in the conventional systems is 2:1 or random.

The description of a system in accordance with the invention which follows is based on an interlace of 4:1 but in practice an interlace of anything from 3:1 up to 9:1 yields superior results. The interface ratio is chosen for each individual requirement, based on many factors, not least of all being the code size and its speed relative to the camera system.

The present system maintains a line scan period of 571us with a modified vertical scan.

There is a total of 128 lines per frame with a 4:1 interlace. This has the effect of allowing the vidicon target a greater time to recover from lag between a line scanned in, say, field I and the same line 4 fields later (see Fig. 3).

This effective decrease in the lag of the camera tube allows an increase of speed of the subject matter bearing the code relative to the camera system, from five metres per minute to in excess of eighty metres per minute.

One means of deriving a 4:1 interlaced frame is represented diagrammatically in Fig.

4.

Within the camera system, a line synchronising pulse exists which initiates a line scan.

This pulse is used to drive a binary counter 32 such that for every 128 synchronising pulses the counter will output its full dynamic range and return to its origin.

The counter output has its least significant 5 bits connected to the most significant 5 bits of a digital to analogue converter 34. This has the effect of producing a full scale linear ramp made up of 32 discrete levels, each level corresponding to one line scan.

If one now considers the effect of the two most significant bits of the counter 32 which are connected to the two least significant bits of the digital to analogue converter 34, the resultant waveform is shown in Fig. 5.

Assuming the counter starts at zero output, a linear ramp of 32 discrete levels is produced by the first 32 line synchronising pulses. At line sync. 33, the least significant 5 bits of the counter 32 will reset to zero whilst the output Q6 of the counter will be set true. This has the effect of setting the least significant bit of the digital to analogue converter 34 for the next 32 line synchronising pulses, hence introducing an offset of one discrete level for that "field".

As two of the most significant bits of the counter 32 are connected in this manner, an offset of 4 levels can be achieved before returning to the initial conditions-thereby obtaining a 4:1 interlace.

This 4:1 interlace, by virtue of being synchronous with the line synchronising pulse, gives a line scan which is truly horizontal.

Thus, the present system is able to improve considerably on the performance of existing systems and enables the vidicon camera to be used in hitherto impracticable situations.

The code will be read provided that it comes anywhere within the field of view of the camera.

The distance the code can be away from the camera is a fraction of the camera lens assembly and can vary from a few millimetres to several metres away.

Among the parameters programmable at the decoding stage can be: (a) code type; (b) number of start and stop bits; (c) number of digits in the code; (d) whether the coding is interleaved or not; (e) ratios of the bar and space thickness; (f) tolerances to which the ratios are defined.

Having read the code, the system can have a number of outputs, for example: (a) a standard computer serial interlace, enabling the unit to pass the code numbers to any host computer for whatever purpose may be required; e.g., product logging, shift reports, and real time stock control, (b) an output to connect the unit to a P.L.C.

(programmable logic controller), enabling the unit to provide product sorting by directing products down different lanes; and (c) an electronically switched output to drive external machines.

Thus, a bar code reader in accordance with the present invention has the advantage that it is designed to read low tolerance printed codes (i.e. the edges of the bars making up the code need not be printed to a high degree of accuracy). As a result, the reader is capable of reading bars made up of a series of blobs. As a further consequence of being able to read low tolerance printed codes, high accuracy printed codes can be read with particular ease. The reader can operate with any format of code, the code format being changed either by the change of a PROM, or is programmable by switch selection. It can thus read quickly and reliably in both the "ladder" and the "picket fence" code orientations.

It can read multicolour codes, including red.

All the processing electronics are located in the camera housing itself.

Claims (13)

1. A bar code reader which operates in real time and which includes a vidicon camera unit adapted to operate with an interlace ratio which falls in the range 3:1 to 9:1.

2. A bar code reader as claimed in claim 1, wherein the interlace ratio is 4:1.

3. A bar code reader as claimed in claim 1 or 2, including analogue processing means for converting the analogue video output of the camera to a digital signal.

4. A bar code reader as claimed in claim 3, wherein the analogue processing means comprises a dynamic analogue slicer which is arranged to maintain an average level to which the analogue video signal is compared to produce a digital output.

5. A bar code reader as claimed in claim 3 or 4, including a pair of discriminators which receive the digitized video output signal and which are adapted to respond to digital levels in said video signal corresponding respectively to bars and spaces in a code viewed by the camera.

6. A bar code reader as claimed in claim 5, wherein a bar is arranged always to correspond to a specific logic level and a space to its complement, whereby the two levels can never be confused.

7. A bar code reader as claimed in claim 5 or 6, wherein the bar and space information, in quantised form, is fed from the discriminators to a decoder which is adapted to interpret this quantised data into the true binary representation of an associated code alphanumeric symbol.

8. A bar code reader as claimed in claim 7, wherein the decoder is a PROM decoder.

9. A bar code reader as claimed in claim 7 or 8, including a verification store coupled to the decoder via a buffer for holding the quantised bar and space data from the discriminators whilst verification checks are made to validate start and stop bits and the quantity of bars and spaces in each character viewed by the camera over a number of "reads".

10. A bar code reader as claimed in claim 9 including means for comparing bar and space width over a number of "reads", means for comparing the recognised number of characters over a number of "reads", means for comparing the start and stop digit information over a number of "reads" and means for comparing the detected code length over a number of "reads", the arrangement being such that if any one of these checks proves false then the system performs another"read" until a consistent result is obtained, but once all the verification checks are "true" a store update signal is outputted to release the information from the verification store.

11. A bar code reader as claimed in any of claims 1 to 10 wherein the line synchronising pulse within the video camera drives a binary counter such that for each of a preselected number of synchronising pulses the counter outputs its full dynamic range and returns to its origin, the binary counter being arranged to drive a digital to analogue converter such as to cause a plurality of successive scans to be offset by one discrete level to achieve interlacing.

12. A bar code reader as claimed in claim wherein, in order to provide interlacing of 4:1 using a total of 128 lines per frame, the counter output has its least significant 5 bits connected to the most significant 5 bits of the digital to analogue converter to produce a full scale linear ramp made up of 32 discrete levels, each level corresponding to one line scan, and the counter has its 2 most significant bits connected to the 2 least significant bits of the digital to analogue converter.

13. A bar code reader substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.