DE68902473T2 - MAGNETIC PRINTING METHOD AND DEVICE FOR CARRYING OUT THIS METHOD. - Google Patents

Description

The invention relates to a method for magnetographic printing (of barcodes) by means of an arrangement of recording and erasing heads.

There are various methods for printing barcodes. The four most common involve

- printing barcodes using an inkjet printer: the resolution corresponds at most to the diameter of the ink droplet, namely 120 um. Another disadvantage is that only a maximum print height of 8 mm can be achieved per print head. In order to obtain a higher barcode, several print heads arranged next to each other are required;

- printing barcodes using a thermal printer: The use of a thermal print head for printing barcodes is known from US Patent 4,661,001. However, such a device does not work particularly quickly in continuous operation, i.e. it hardly prints more than 10 cm/s. In addition, thermal printing is not very durable in the long term;

- printing barcodes using a laser printer: This method cannot be used continuously;

- printing barcodes using a raster printer: the resolution is very low at 250 um and the print quality is inadequate for certain applications.

However, there are also various devices with read/write and erase magnetic heads, as described for example in FR-PS 2 583 550. In this patent, which relates to the reading of credit cards and to the detection of fraudulent misuse of these cards, after a first pass of the card over the read head, the personal identification number is compared with the card number and with a list of rejected numbers. If the card has been misused, then at least one track on the card can be erased during a second pass of the card, thus rendering the card unusable. However, this patent does not indicate the simultaneous use of an erase and a write head on one and the same magnetic support, as is the case with the present invention.

US-A 3,735,416 describes a magnetic printing system that includes means that work with the printing medium to store or erase information on the medium. However, such a system cannot create a latent image consisting of two rows of information that at least partially overlap in one pass.

These disadvantages of the prior art are remedied in the present invention by

- different information can be printed on a magnetic tape by means of an arrangement of write and erase magnetic heads after the magnetic tape has moved past these heads once;

- a continuous magnetographic printing process is possible at a higher speed than in the prior art (according to the invention, speeds of about 1 m/s are achievable), the speed being in particular between 20 cm/s and 100 cm/s;

- a print is created that is very durable and has a good graphic resolution (approx. 75 um).

Further objects of the invention are explained in the following description.

The invention relates to a method for producing a latent image on a magnetic tape by means of a plurality of erasing and recording magnetic heads, which is characterized in that in order to produce a latent image consisting of two groups of information which at least partially overlap and are recorded during a single movement of the tape past the heads, in several steps

a) a first type of information is recorded on a defined area of the magnetic tape,

b) in the overlapping area of the two groups of information, at least part of the information recorded during step a) is deleted, and

c) a second type of information is recorded, at least part of which is located in the area erased during step b).

The following description refers to a device for printing barcodes according to the EAN (= European Article Number) standard, in which the method according to the present invention is used. This method can of course also be used in other applications. The description is explained in more detail using the drawings.

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Fig. 1 shows an example of the type of barcodes that can be achieved using the method according to the invention;

Fig. 2 is a block diagram of the device for carrying out the method according to the invention. (For clarity, each part of the device corresponding to the individual stations has been shown);

Fig. 3 shows a possible arrangement of magnetic heads for implementing the method according to the invention;

Fig. 4 is a diagram showing how the signal applied to the writing heads is generated;

Fig. 4a is a representation of the form of the signals for controlling the magnetic heads;

Fig. 5 is a schematic representation of the different steps for generating the modulation signal which is superimposed on the bias signal for generating the write signal;

Fig. 6 is a diagram showing how the signal applied to the erase heads is generated;

Fig. 7 is a diagram of the system for controlling magnetic heads;

Fig. 8 shows how the label memory (page layout plan) is organized; and

Fig. 9 is a simplified representation of the processes that generate the magnetic head control signals.

Fig. 1 shows examples of barcodes that can be produced using the printing method according to the invention.

Throughout the description, the term "control" is used in the sense of "monitor".

Barcodes, which are increasingly used, consist of rows of bars (generally 0.5 - 2.5 cm high and 200 - 500 µm wide) with numbers arranged below them. According to the EAN standard, an additional number is placed in front of the barcode, which is aligned with the numbers indicating the type of printed barcode. In one embodiment, which is explained in more detail below, a device is provided with which alphanumeric characters can be written over the code, eg advertising information.

In general, the sets of bars forming the code are delimited by a lateral separator consisting of two bars, which is generally thinner and longer towards the bottom than the other bars and which partially encloses the numbers arranged below the code. Accordingly, a central separator consisting of two bars is provided, which is identical to the lateral separators.

These barcodes contain information about the brand owner and the manufacturer’s article code and are either printed on a label that is stuck onto the product or directly on the product packaging.

The barcode is read using an optical reading pen or laser, which generates a current when it is scanned over the code. This current is a function of the reflected light and is proportional to the width of the white and black bars. Any error that occurs when printing a barcode (density, width) inevitably leads to a reading error. Therefore, it is crucial that the bars are printed precisely.

Fig. 2 shows a simplified representation of a device that is suitable for carrying out the method according to the invention.

This device includes the following parts:

a) a magnetic tape 1 designed as an endless belt, which is a motor in the form of a pulley 2 is driven in the direction of arrow A; the belt is guided by rollers which are provided at several points along the transport path of the belt;

b) a paper tape 3 on which the barcode is printed. This tape is unwound from a reel 4, then driven by a motor in the form of a pulley, passed in front of the transfer station 9 and the fixing station 10 and then wound onto a take-up reel 5;

c) a write/erase station 6, which comprises the group of magnetic heads and is explained in more detail below. In this station, field gradients are formed on the magnetic tape, which produce a latent image that can be developed using a toner;

d) a development station 7 with a toner supply in contact with a rotatably mounted drum 8, which is equipped with an odd number of fixed magnets on a portion of its inner circumference. The particles are transported by the magnetic portion of the drum and, in the absence of magnetization on the magnetic tape, are released when the latter comes into contact with the drum. The toner particles are then attracted by field gradients on the magnetic tape. This station contains a device which generates vibrations on the carrier (for example a piezoelectric device) and with which the particles deposited between the images to be developed can be removed;

e) a transfer station 9 in which the magnetic tape is in contact with the paper tape. In this station, if possible, all toner particles are transferred from the magnetic tape to the paper tape by pressure medium;

f) a fixing station 10 in which the toner particles are permanently fixed to the paper belt;

g) a cleaning station 11 comprising a magnetic brush with a rotating core and a mechanical brush to to demagnetize the belt and remove the remaining toner particles. The device is then ready for a new printing cycle.

Fig. 3 shows an arrangement of magnetic heads for continuing the method according to the invention. This arrangement of magnetic heads comprises:

- a print head WR0 which prints barcodes;

- an eraser head Er0 which erases part of the bars so that the desired numbers can be inserted under the code. This eraser head is controlled so that the lateral or central separating bars are not erased;

- a first group of seven writing heads arranged as follows:

- a first row D1 consisting of four writing magnetic heads (WR1-WR4) recording four parallel tracks;

- a second row D2 not aligned with D1 and comprising three write magnetic heads (WR5-WR7) arranged in a row offset from the magnetic heads in row D1.

This group of writing heads allows numbers to be written under the barcodes.

- a second group of seven print heads arranged in the same way as the first group, namely:

- a first row D'₁ consisting of four writing heads (WR₈-WR₁₁);

- a second row D'₂, not aligned with D'₁ and comprising three write heads (WR₁₂-WR₁₄) arranged in a row offset from the magnetic heads in row D'₁.

This group of writing heads allows numbers to be written under the barcodes.

The device for controlling these 16 magnetic heads (15 of which are write heads WR�0;...WR₁₄ and one is an erase head ER�0) is explained in more detail below.

The above arrangement does not have to be limited to this number of magnetic heads. Of course, a further erasing head can also be provided above the first erasing head, which can be moved in the transverse direction and can change the height of the code if necessary by erasing the upper part of the barcode.

It is also possible to provide an erase head extending across the entire width of the magnetic tape, which is arranged in front of the above-mentioned group of magnetic heads and which erases the recordings on the magnetic tape before new data is written to the tape.

The following figures show how the signal applied to magnetic heads is created.

In this context, however, it is important to distinguish between write heads and erase heads.

Fig. 4 refers to writing heads (WR�0 - WR₁₄). One embodiment of the method according to the invention involves recording in the horizontal direction (as opposed to recording in the vertical direction). The average diameter of the toner particles is 25 µm and the wavelength of the recorded signal is 100 µm. Of course, different wavelengths must be used for particles of different sizes, because the best conditions for optimal development are when the wavelength is about three times as large as the particles. Depending on how wide the desired line is to be, 1, 2...n wavelengths are recorded. Since barcodes consist of a series of more or less fine lines which can be very close to each other, it is crucial to record flawless sinusoids with very precise wavelengths on the magnetic tape. This precision is absolutely necessary for the recorded signal in order to be able to read the code accurately later. The signal transmitted to the writing heads is obtained by superimposing two signals:

- a first bias signal Vbias for linearising the recording material and thus improving the quality of the recorded signal and the resolution of the bar code. This bias signal, whose frequency is around 80 kHz and whose peak-to-peak amplitude is in the range 200 to 300 V, is obtained by means of a conventional power stage;

- a modulation signal Vmod whose frequency Fmod is a function of the tape speed and lies between 100 Hz and 10 kHz and whose peak-to-peak amplitude varies between 2 V and 15 V.

Fig. 5 shows how the modulation signal is generated. An optical encoder 16, which is mounted on the pulley driving the magnetic tape, generates a sampling frequency fsamp as a function of the tape speed V according to the relationship fsamp = kV, where k is a constant. This sampling frequency is divided by 16 in a 4-bit counter 17. The modulation frequency is therefore given by the equation fmod = fsamp / 16. This signal is fed into the wave-shaped memory (in this case a sinusoidal wave), then converted to analogue by means of a digital-to-analogue converter 19 and then passed through a low-pass filter 20 to finally form the modulation signal Vmod; after this signal formation, a pre-correction is carried out to modulate the signal as a function of frequency in order to obtain a constant magnetisation regardless of the signal frequency. This produces a clearly sinusoidal signal whose wavelength is calculated as follows:

By carefully selecting the proportionality constant k, it is possible to obtain exactly the desired wavelength, regardless of the belt speed, in the case of the present embodiment, 100 µm.

With reference to Fig. 4, it will now be explained how the bias signal Vbias and the modulation signal Vmod are combined. For each write head (WR�0 - WR₁₄), the modulation signal Vmod is added to the bias signal Vbias by means of a notch filter 12. This notch filter is a circuit which applies an infinitely high impedance to the bias signal and a very low impedance to the modulation signal. The modulation signal is therefore not affected by the bias signal. Each write head has a modulation signal controlled by analog switches 13, and the analog switches are in turn controlled by the signals generated by a computer.

When the switch controlled by Ai (0=< i< 14) is on, the other switch controlled by i is off. In this way, the desired number of wavelengths is transmitted to the write heads. In contrast, if no signal is to be transmitted to the write head, the switch controlled by Ai is turned off while the switch controlled by i is turned on.

Fig. 4a shows this process using the example of the write head WR�0.

Fig. 6 shows how the signal transmitted to the erase head ER�0 is generated.

A continuous signal of about 50 mA is sufficient for this. As with the writing heads, this signal transmitted to the erasing head is also controlled by a system of analog switches 22, which in turn are controlled by signals generated by a computer.

If the switch controlled by the ₁₅ signal is on, then the switch controlled by A₁₅ is off. Thus, the erase signal is transmitted to the erase head. In the opposite case, no signal is transmitted to the erase head.

According to Fig. 4, 4a, 5 and 6, fifteen write heads and one erase head must be controlled. For this, sixteen control signals A₀-A₁₅ must be generated. The complementary signals A₀-A₁₅ are supplied directly by an inverter.

Fig. 7 shows the system used to control the magnetic heads.

The most important means for carrying out the method according to the invention can be divided into three groups:

1) a software 25 consisting of a basic program and an assembly program. The basic program provides the dialogue between the operator and the system (menu display and parameter entry). The parameters entered are sent to a memory area to which the assembly program has access. The assembly program retrieves these parameters and creates an image, i.e. a binary representation of the label to be printed.

2) Devices for communication with the outside, namely

a) for communication with the operator. The basic program asks the operator to enter 26 different printing parameters via the keyboard. In a preferred embodiment, the operator is asked to enter the code itself and the data to be printed via the code. possible additional information, the selected wavelength, the width of the module (where the module represents the unit of width of the bars that make up the code, ie the thinner bar desired) and the length of the label; and

b) for communication between the computer and the magnetic heads. This communication is ensured in the computer/magnetic heads direction by means of two 8-bit outputs 27, 28 of a peripheral interface adapter, through which the data to be printed is transmitted, and in the opposite direction magnetic heads/computer by means of a control signal which indicates that the magnetic heads are ready to receive new information. Other variants are also possible, for example signals for controlling drive motors, the tension of the belt and the toner supply.

3) a read-write memory, which includes the following devices

- a font memory 29 containing an ASCII code for the characters and a printing code. This memory is used to display the label;

- a label memory 30 containing a binary representation of the label to be produced. Fig. 8 shows how this memory is organized. To make the relationship clearer, this area is shown in the form of two adjacent 8-bit areas, one of which corresponds to even-numbered addresses (Ei-1 word) and the other to odd-numbered addresses (Ei word). The higher-order bit of the first area corresponds to the signal that controls the write head WR�0, while the higher-order bit of the second area corresponds to the signal that controls the read head ER�0; the other seven bits of the first and second areas control the write heads WR₈-WR₁₄ and WR₁-WR₇, respectively.

In one possible embodiment, the length of the label is divided into sections whose width corresponds to the selected wavelength, e.g. 100 µm. Each section of a 100 µm label is thus associated with two 8-bit words stored at two consecutive addresses of the label memory (one address being even and the other odd). Both words contain the control signal applied to each of the sixteen magnetic heads within a 100 µm section. Furthermore, since the magnetic heads are offset by an integer number of wavelengths, this offset must be taken into account in the binary representation of the label. For example, the D'₁ and D'₂ bars are shown to be spaced apart by a distance d. In order to obtain carefully printed alphanumeric characters, the D'₂ and D'₂ bars must be spaced apart by a distance d. the data in question are offset by a number of addresses equal to d/100 um. All operations related to the image of the label are carried out using the assembly program.

Fig. 9 is a simplified diagram showing the processes involved in generating the control signals for each magnetic head. The various printing parameters entered by the operator are provided by the basic program which transfers the parameters to an area accessible to the assembly program. This program uses the parameters to create a binary representation of the label to be printed, starting from the first address of the label memory. Then, when the appropriate command is given (when the control signal = 0), the microcomputer starts from the last address in the label memory and looks up the word Ei which contains the signals for controlling the erase head ER₀ and the write heads WR₁-WR₇, places them on the A output of the peripheral interface adapter, then looks up the word Ei-1 which contains the control signals for the write heads WR₀ and WR₈-WR₁₄, and places them on the B output of the peripheral interface adapter. The words are then the various channels which feed the corresponding magnetic heads. This process is repeated until all the data on the label has been recorded. As soon as the latent image corresponding to one label has been recorded, the latent image corresponding to the next label is recorded, and so on. At the same time, the tape is advanced so that the latent image can pass through the various stations of the device until finally a visible image is transferred to the paper tape 3.

The method according to the invention can therefore be carried out using the device described above.

If, for example, barcodes of the type explained in Fig. 1 (EAN standard) are to be printed using the arrangement of the magnetic heads described in Fig. 3, the process is as follows:

a) the latent image of the first character indicating the type of bar code to be printed is recorded by a series of writing heads WR₁-WR₇,

b) the print head WR0 is controlled so that it prints the bars forming the code,

c) the erase head ER�0 is controlled to erase at least the writing area 14 common to the heads WR�0 and WR�1-WR�7, without affecting the lateral and central separating lines, which must retain their original length,

d) the latent image of the numbers below the barcode is formed using the write heads WR₁-WR₇ and the latent image of the additional information above the barcode is formed optionally using the write heads WR₈-WR₁₄.

Claims (6)

1. Method for producing a latent image on a magnetic tape by means of a plurality of erasing and recording magnetic heads, characterized in that in order to produce a latent image consisting of two groups of information which partially overlap and are recorded during a single movement of the tape past the heads, in several steps a) a first type of information is recorded on a defined area of the magnetic tape, b) in the overlapping area of the two groups of information, at least part of the information recorded during step a) is deleted, and c) a second type of information is recorded, at least part of which is located in the area erased during step b). 2. Method according to claim 1, characterized in that the information recorded during step a) is barcode information. 3. Method according to claim 2, characterized in that the first information group contains control information which is longer than the information itself and which is not modified by the deletion taking place during step b). 4. Method according to claim 1, characterized in that the information recorded during step c) is alphanumeric information. 5. Method according to claim 1, characterized in that the latent image is created in a continuous process at a speed of between 20 cm/s and 100 cm/s. 6. Device for carrying out the method according to one of claims 1 to 5, with at least three magnetic heads, which consist of at least one erasing and two recording magnetic heads, characterized in that the erasing magnetic head is arranged so that it erases the recording area (14) which is common to at least two recording magnetic heads.