DE3723348A1 - Method and system for reading bar codes - Google Patents

Abstract

The invention proposes a method for reading bar codes, the bar code being illuminated by means of light, preferably in the visible or IR range, and reflected light being detected and the transitions between areas of different reflectivity of the bar code, i.e. between blackened bars and free spaces between the bars of such a code, being detected and their distances being determined, and a system for reading bar codes, the bar code being illuminated by means of light, preferably in the visible or IR range and reflected light being detected and converted into an electrical signal and a differentiator being provided for differentiating the received and preprocessed, and at least one comparator, which signal ensures reliable evaluation in a simple manner due to the fact that the received signal is diffferentiated only once, that the time of the positive and negative peak value of the differentiated signal is determined and by means of this a corresponding squarewave signal is generated and that the differentiator is in each case followed by a detector for a positive peak value and for a negative peak value of the differentiated signal and a comparator in each case follows these and directly follows the differentiator.

Description

The invention relates to a method for reading bar codes, the bar code using light, preferably in the visible or IR range illuminated and reflected light detected and the transitions between areas are different Reflectivity of the bar code, d. i.e., between blackened Lines and spaces between the lines of such  Codes, recorded and their distances are determined and a System for reading bar codes, the bar code using Light, preferably illuminated in the visible or IR range and detects reflected light and converts it into an electrical one Signal is converted and a differentiator to differentiate adorn the received and preprocessed signal and min at least a comparator is provided.

From the German patent. . . (Patent application P 36 01 083.9) are a generic method and a generic Device known that the state of the art improve that they have high illuminance as well high immunity to electromagnetic interference in the area of signal recording as well as a high signal-to-noise ratio enable ratio at low cost. For a safe exit evaluation of the analog signal recorded by the photo element made a two-fold differentiation of the signal the time or range of the steepest edge of the analog signals - as a representation of the transition of a bright area to a dark bar of a bar code or vice versa - in the context of a zero value comparison, whereby continue it is ensured by thresholds that only flanks go sufficient steepness can be determined.

The invention has for its object at a similarly high Detection reliability and insensitivity to electromagnetic table disturbances and against noise a simplified procedure to propose for the evaluation of the recorded analog signal.

According to the invention, the stated object is achieved in a method of the type mentioned, solved in that the received Signal is differentiated only once, that the times of the positive and negative peak of the differentiated Si gnals determined and by this a corresponding rectangle signal is generated.  

The invention therefore does not take a second differentiation of the ana log signal before a zero value detection of the doubly dif referenced signal. It will be a second dif ferentiation saved and only the peak or maximum value of the analog signal differentiated only once, this being done by changing the differentiated signal its respective value as long as the signal increases, is held and when reducing (the absolute value) of the differentiated signal compared to the recorded A signal is emitted, causing the overturning point and hence the location of the peak value of the differentiated signal is detected. This will work for both the negative as well for the positive peak value, whereupon by a rectangular waveform is formed which corresponds to the of the scanned or recorded bar code. The rectangular signal is preferably created by that through the positive peak of the differentiated signal set a flip-flop and this through the negative tip of the differentiated signal is reset. To sources of error, for example, by roughness of paper or the like switch, it is provided in a further embodiment that at least the positive peak values of the differentiated signal be detected above a predefinable threshold. Too To be able to examine low quality barcodes, for example wise barcodes where the blackening of the black bars one made before by matrix printers preferred embodiment that a reset by dense swan which is excluded from a bar of a bar code. To disturbances due to light fluctuations or the like switch off can be provided in a further embodiment, that the barcode be with high-frequency, modulated light is lit and the same high from the received light frequency portion is selected and that high frequency and low-frequency interference can be filtered out. A continuation  tion also provides that the received signal ver is strengthened.

A generic system for solving the above. Task and in particular for the implementation of the method provides that the differentiator a detector for a positive Peak value or a negative peak value of the differentiated Signal is subordinate and this as well as the difference a comparator is arranged in each case, with Er Generation of the square wave signal is provided in particular that a flip-flop is connected downstream of the comparators. To the formwork malfunctions due to paper roughness or same, the system according to the invention is designed such that the differentiator is followed by a device that at least only a sufficiently steep rising flank (there sufficient positive peak value of the differentiated signal) of a received signal for further processing, in particular the establishment of a voltage divider Creation of a predeterminable threshold value. Um how mentioned above, even with barcodes of poor print quality a reliable detection of the end of a blackened stroke a barcode and only then, but not a reset in the event of fluctuations in blackness in the stroke to cause - as long as the flip-flop is not reset is a new bet due to the blackness fluctuations of given rising signal edges harmful - a preferred further training provides that the Comparator for the negative peak value a link circuit is assigned. The system sees in training to turn off stray light in particular that the Barcode illuminating light source using a high frequency generator for frequency modulation of the light is upstream and that the receiving device one of the high-frequency modulation has adapted selective amplifier and furthermore, that the preamplifier has filters.  

Further advantages and features of the invention result from the claims and from the following description in which an embodiment of the invention with reference to the drawing explained in Einhelnen. It shows

Fig. 1 is a schematic block diagram for explaining the method according to the invention;

Fig. 2 is a concrete circuit of the system according to the invention; and

Fig. 3a-d are diagrams of the signals occurring.

The mechanical structural design of the fiction system, especially the structure of barcode reading head is the same as for patent application P 36 01 083.9, so that the relevant disclosure in full reference is made to the aforementioned application.

The bar code reading head has a light transmitter 21 with a light-emitting diode 6 . The light reflected by a bar code enters an optical waveguide, which leads to a photo element 15 , which carries out the opto-electrical conversion of the bar code signals recorded and is connected to a receiver and amplifier circuit 22 .

Although the method according to the invention provides a high level of transmission security with constant light, it can be switched off completely and safely in order to disrupt interference from daylight and to further improve the signal / noise ratio, the light source can be the light-emitting diode 6 , optionally with an AC signal of a frequency operated in the range of F = 10 to 30 KHz. In this case, the receiver and amplifier circuit 22 works selectively at a center frequency which speaks to the generator frequency of the light emitting diode.

The receiver and amplifier circuit is followed by a differentiator 31 , which in turn is followed by a peak value detector 32 for the positive peak value of the signal differentiated by the differentiator 31 and a peak value detector for the peak value detector 33 for the negative peak value of the differentiated signal. The peak value detectors 32, 33 are each followed by a comparator 34, 36 which compare the outputs of the respective peak value detectors 32, 33 with the direct output of the differentiator 31 and emit a corresponding signal. The comparator 34 generates a set signal for a flip-flop 37 . The output signal of the comparator 36 is linked to the direct analog signal generated by the receiver and amplifier circuit 22 in a logic logic circuit 38 , which only generates a reset signal for the flip-flop 37 . By setting and resetting the flip-flop 37 , a TTL output signal is generated that corresponds to the bar code recorded by the photodiode 15 and can be further processed, for example, in a computer.

Fig. 2 shows a concrete circuit. In a power supply unit, not shown, the non-inverting input of an operational amplifier can be connected to a voltage divider. The output voltage of the operational amplifier, which works as an impedance converter, is preferably + 2.5 volts. This voltage forms the reference ground for electrical signal processing.

The light transmitter 6 (IR-LED) is connected to the operating voltage of + 5 volts via a series resistor. The size of the series resistor depends on the length of the fiber optic cable or the resistance of a copper cable that is carried along. The transmission current for the transmitter is 30 mA, for example.

As a sensor, a photo element 15 is used , which was located at a counter at the inverting input of an inverting operational amplifier OP 2 . The output voltage from OP 2 reaches the input of a non-inverting amplifier OP 3 . With the potentiometer P 1 , the gain of this level can be set. The differentiator 31 with the operational amplifier OP 4 differentiates the output signal from OP 3 . The differentiating signal is at the inputs of the two peak value detectors for the positive and negative peak value of the differentiated analog signal.

The operational amplifier OP 5 has in its negative feedback branch a diode D 1 , via which the charging capacitor CL is charged and stored. If the voltage at the output of OP 5 is again lower than the voltage present at the charging capacitor CL , the diode D 1 blocks and the discharge of CL takes place only slowly via the resistor RP connected in parallel.

The capacitor voltage is via an impedance converter OP 6 at the non-inverting input of the comparator 34 ( OP 7 ), which compares the capacitor voltage with the differentiated analog signal itself. The voltage at the charging capacitor CL is always lower than the differentiated signal itself due to the predetermined time constant (by CL and RP) during the positive rise of the differentiated signal, whereby the comparator OP 7 initially remains at "low". If, after reaching its peak value, the differentiated analog signal is less than the voltage at the capacitor CL , the comparator OP 7, 34 switches from "low" to "high". This sets the starting flip-flop.

The diode D 2 serves to ensure that the charging capacitor CL is discharged far enough before the next positive peak value. The diode D 2 becomes conductive when the differentiation signal by the forward voltage of the diode D 2 is less than the capacitor voltage at the charging capacitor CL .

So that roughness of the paper, and associated voltage changes at the output of OP 5 do not cause charging of the charging capacitor CL of the peak value detector, the charging capacitor CL is at a low positive potential with respect to the reference ground for the operational amplifier via a voltage divider. The differentiated analog signal must therefore be greater than this threshold value in order to switch the comparator OP 7 from "high" to "low".

The negative peak detector 33 operates in the same way as the positive peak detector 32 . On the Spanunngs divider for the response threshold may be omitted, since a reset pulse for the output flip-flop 37 only causes a change in its condition when the flip-flop has been previously set 37th The comparator signal of the comparator 36 (OP 10 ) is differentiated with an RC element and reaches the input of a comparator OP 12 . This generates the reset pulses for the output flip-flop. Since not only the transition from black to white of the bar code could generate a reset pulse at the input of OP 12 , but also the edges of the individual matrix printer dots, a logic circuit 38 with an operator OP 12 is also provided as a logic link. A comparator 11 (OP 11 ) generates a square wave signal from the analog signal from OP 3 of the amplifier and an adjustable threshold value, which controls the inverting input of OP 12 via a voltage divider. If there is no analog signal via the high pass at the plus input of OP 11 , the negative potential at the minus input of OP 11 means that its output is high. Via a voltage divider, the minus input of OP 12 is at a more negative potential than the plus input of OP 12 , which has ground potential. The output of OP 12 is therefore high.

If an analog signal is present via the high pass at the plus input of OP 11 , the negative parts of the analog signal switch the output of OP 11 to low. The voltage at the minus input of OP 12 thus becomes even more negative than the reference ground at the plus input of OP 12 . Negative voltage peaks at the plus input of OP 12 cannot change the output state of the comparator. If the analog signal at the plus input of OP 11 becomes more positive than the potential at the minus input of OP 11 , the output of OP 11 becomes high and the minus input of OP 12 becomes more positive. But even now the minus input of OP 12 is more negative than the plus input of OP 12 . The output of the comparator remains high. A negative pulse at the plus input of OP 12 can now switch the output of the comparator to low for the duration for which the pulse is more negative than the minus input, and thus generate a valid reset pulse for the output flip-flop 37 .

The output of OP 12 then provides the reset pulses for the output flip-flop 37 . At the Q output of the flip-flop there is a TTL signal that corresponds to the bar code pattern.

The signal emitted by the photoreceptor is thus amplified 22 in the vorver and then differentiated in the differentiator 31 . The differentiated signal is fed to the peak value detectors 32, 33 . A positive peak value generates a set signal for the flip-flop 37 in which a rising edge of the differentiated signal stores the respective value and in the comparator the set pulse is emitted from its stored value when the instantaneous value of the differentiated signal falls below. Only a differentiated signal above a predetermined threshold, ie an edge of sufficient steepness, is taken into account.

The signal from the comparator 36 and peak detector 33 for the negative peak value is combined by the logic circuit 38 with a square wave signal formed from the direct analog signal and a predetermined threshold value in order to form the reset pulse for the flip-flop 37 . This avoids that blackening fluctuations within a bar code line cause reset pulses and ensures that such only occur at the end of a bar code line.

In Fig. 3 the bars of the bar code are shown schematically, the rectangles representing the black lines of the bar code and in between the bright, unprinted positions of the wearer. In Fig. 3b, the analog signal A picked up and amplified by the photodiode is shown, with E indicating a drop in the analog signal due to the resolution of matrix printer pixels by the reading pen, while R indicates a change in the analog signal due to the roughness of the paper.

The Fig. 3c shows the analog signal by differen tiation as differentiated signal D. Further, the data stored by the charging capacitors positive and negative voltages PS, NS and the threshold ST are in the Fig. 3c.

FIG. 3d finally shows the TTL signal achieved according to the invention, which corresponds to the barcode signal BC of FIG. 3a.

It can be seen that the signals at the charging capacitors each initially almost follow the differentiated signal up to their peak value and, when the differentiated signal is below the capacitor voltage, the respective comparator pulse for setting or resetting is triggered by the respective comparator, which also, if there is a real transition in the reflectivity of the bar code BC , the flip-flop has to be switched.

In the area E of the dip in the analog signal due to the resolution of matrix printer pixels by the reading pen, there is a turning point of the differentiated signal with two relative extremes of the differentiated signal at E 1 . The comparator 36 , OP 10 emits a reset pulse at E 2 , but the operator OP 12 does not reset the output flip-flop due to the fundamentally negative analog signal value at E and therefore causes the TTL value to decrease to zero.

In the area of the paper roughness R of the analog signal A ( FIG. 3) there is also an area R 1 of relative extremes of the differentiated signal D. However, paper roughness generally only requires gentle signal changes, so that the relative maximum remains below the threshold value ST to be set - while dips due to the resolution of matrix printer pixels may cause “steep” extremes, so that there is no setting of a simple threshold value for the differentiated signal possible if this is sufficient for paper roughness. Since the output TTL value is low or zero anyway, the crossing of differentiated signal and voltage at the charging capacitor in the branch of the negative peak value, as indicated at R 2 , does not distort the output TTL signal.

Claims (15)

1. A method for reading bar codes, wherein the bar code is illuminated by means of light, preferably in the visible or IR range, and reflected light is detected, and the transitions between areas of different reflectivity of the bar code, that is, between blackened lines and spaces between the lines of a such codes, detected and their distances are determined, characterized in that the received signal is differentiated only once, that the times of the positive and negative peak value of the differentiated signal is determined and a corresponding square-wave signal is generated by this. 2. The method according to claim 1, characterized in that at least the positive peaks of the differentiated Signals are detected over a predetermined threshold. 3. The method according to claim 1 or 2, characterized in that a reset due to density fluctuations in the density is excluded in a bar of a bar code. 4. The method according to any one of the preceding claims, characterized characterized by the positive tip of the differen decorated signal set a flip-flop and this through the negative peak of the differentiated signal returned is set. 5. The method according to any one of the preceding claims, characterized characterized in that the bar code with high-frequency, mo light is illuminated and received from the Light the same high-frequency portion is selected. 6. The method according to any one of the preceding claims, characterized characterized that high-frequency and low-frequency Interference can be filtered out. 7. The method according to any one of the preceding claims, characterized characterized in that the received signal is amplified. 8. System for reading bar codes, the bar code being illuminated by light, preferably in the visible or IR range, and reflected light being detected and converted into an electrical signal, and a differentiator ( 31 ) for differentiating the received and preprocessed signal and at least one comparator is provided, characterized in that the differentiator (31) each comprise a detector (32, 33) for a positive peak or a negative peak value is arranged downstream of the differentiated signal, and these as well as directly to the differentiator (31) each comprise a comparator (34, 36 ) is subordinate. 9. System according to claim 8, characterized in that the comparators ( 34, 36 ) is followed by a flip-flop ( 37 ). 10. System according to claim 8 or 9, characterized in that the differentiator ( 31 ) is arranged downstream of a device that lets at least only a sufficiently steep rising edge (sufficiently positive peak value of the differentiated signal) of a received signal for further processing. 11. System according to claim 10, characterized in that the Set up a voltage divider to create a front definable threshold. 12. System according to any one of claims 8 to 11, characterized in that the comparator ( 36 ) for the negative peak value is assigned a logic circuit ( 38 ). 13. System according to one of claims 8 to 12, characterized in that the bar code illuminating light source ( 6 ) is preceded by a high frequency generator for frequency modulation of the light. 14. System according to claim 13, characterized in that the  Receiving device one of the high-frequency modulation matched selective amplifier. 15. System according to claim 14, characterized in that the Has preamplifier filter.