EP0249523A1 - Method of and apparatus for operating a thermal printing head - Google Patents

Abstract

a series-type thermal print head is controlled on the fly, during a series of cycles of duration at most equal to the printing duration of the dots, to be able to be heated independently during the first half and the during the second half of each cycle. It is thus possible to print a dot offset by half the length of a normal dot, or an extended dot, one and a half times longer than the normal dot, to improve the definition of the print without reducing the writing speed. The invention applies to printing systems, in particular for a printer connected to a word processing device.

Description

The present invention firstly relates to a method of controlling a print head, or writing, thermal, of the serial type, for a printing system, in particular for a printer connected to a word processing device. .

A thermal writing head generally comprises a plurality of heating elements, in general resistive elements arranged to be traversed by a current, and to cooperate either directly with a printing medium, heat-sensitive paper, or indirectly with a medium. ordinary printing, by means of a ribbon coated with an ink melting with the heat of the elements. In the first case, it is a direct thermal printing, in the second case, a thermal transfer printing.

This is a series type thermal printing, and in this case, the movable head comprises at least one bar, generally vertical, of elements, aligned in a direction orthogonal to the direction of rectilinear displacement, during the printing of a line of graphics, corresponding to the height of the bar, of a head support carriage.

• - on entraine ladite tête en déplacement longitudinal le long d'une ligne à imprimer, à une vitesse déterminée,
• - les éléments chauffants étant soumis à des cycles thermiques comprenant, chacun, une durée de chauffage initial et une durée de refroidissement pour imprimer des points.

More particularly, the invention relates to a method for controlling a serial type thermal writing head for a printer, provided with a strip of heating elements, in which,

• said head is driven in longitudinal displacement along a line to be printed, at a determined speed,
• - The heating elements being subjected to thermal cycles each comprising an initial heating time and a cooling time to print dots.

In this process, the set of heating elements that are heated during the same cycle obviously depends on the configuration of the transverse section of the line to be printed, of dimension L, equal to the product of the speed of displacement by the duration T of the cycles.

In such a method, and taking place for example in the extremely frequent case where the line is written horizontally, the vertical definition depends only on the size and the spacing of the heating elements of the writing head. On the other hand, the horizontal definition is approximately inversely proportional to the speed of training of the head. In fact, taking into account the thermal inertia of a heating element, it is not possible to make the interval of time during which it is hot enough to print as small as desired. This time interval is therefore bounded by a lower limit, the horizontal dimension of the "point" that it prints is therefore substantially inversely proportional to the speed of drive of the head.

In the process defined above, a compromise must therefore be made between the horizontal definition and the speed of writing the head.

The present invention aims to provide a better compromise, that is to say, at constant writing speed, better horizontal definition or, with equivalent horizontal definition, greater writing speed.

To this end, the present invention relates to a method of the type defined above, characterized in that the duration of the cycles is chosen at most equal to the duration of printing of the dots and that some of the heating elements are subjected to heating for a period starting in the middle of the cycles.

In the case where a heating element is subjected to an initial heating for a non-zero duration, the invention pre therefore sees being able to subject it, within the same cycle to a second heating.

When the heating element is subjected to two successive heatings during the same cycle, it prints an "extended point". The start of this extended point coincides with the start of a normal point, but its length is greater than that of the normal point.

On the contrary, if the initial heating time of an element is zero, within a cycle, the invention provides that it can only be subjected to heating from the middle of the cycle. In the latter case, the heating element prints an offset point. The start of this offset point is offset by half the width of a slice with respect to a normal point.

It can therefore be seen that, without reducing the head drive speed, it is capable of printing on the fly, in addition to a normal point, an offset point and an extended point. This makes it possible to obtain an improvement in the horizontal definition of each line, which gives a better print quality, in particular with regard to the reproduction of the slashes frequently encountered in alpha-numeric characters. Taking into account that these are more slightly inclined with respect to the vertical than with respect to the horizontal, the fact that the vertical resolution remains unchanged, and the fact that the horizontal dimension of the point is not reduced have a little noticeable influence on the final quality of the print.

In the preferred implementation of the method of the invention, the initial heating time and the heating time starting in the middle of the cycle are less than half the duration of a cycle.

In this case, the offset point has the same length as the normal point and the extended point a length substantially equal to one and a half times the length of the normal point. The possible offset being equal to half a normal point length, we can say that we obtain a "pseudoresolution" double of the original resolution. A true double resolution would indeed suppose that the length of the point is divided by two, which is not the case.

Advantageously, the duration (TI) of heating, during a first half-cycle, of a heating element determined is modulated so as to shorten this duration if this heating element has been heated during one of the two preceding half-cycles, and modulating the duration (TID) of heating, during a second half-cycle, of a determined heating element so as to shorten this duration if this heating element has been heated during one of the three preceding half-cycles.

In another implementation of the method of the invention, the writing head is provided with a first bar and a second transverse bar arranged one after the other in the longitudinal direction, and in which, at during an odd-order cycle, the heating elements of the second strip are not subjected to any heating, and, during an even-order cycle, the heating elements of the first strip are not subjected to any heating .

• - des moyens pour entrainer ladite tête en déplacement longitudinal le long d'une ligne à imprimer, à une vitesse déterminée,
• - des moyens de chauffage desdits éléments chauffants,
• - des moyens de calcul de la durée du chauffage commandant lesdits moyens de chauffage, pour soumettre lesdits éléments chauffants à des cycles thermiques comprenant, chacun, une durée de chauffage initial et une durée de refroidissement, pour imprimer des points,
  dispositif caractérisé par le fait que
• - les moyens de calcul comprennent des moyens pour ajuster la durée des cycles à une durée au plus égale à la durée d'impression des points, des moyens pour déterminer certains éléments chauffants devant être soumis à un chauffage à partir du milieu des cycles, et sont agencés pour commander lesdits moyens de chauffage pour soumettre lesdits éléments chauffants déterminés à un chauffage pendant une durée commençant au milieu des cycles.
  L'invention sera mieux comprise à l'aide de la description suivante de la mise en oeuvre préférée du procédé, et de la forme de réalisation préférée du dispositif de l'invention, faite en se référant aux dessins annexés, sur lesquels,
• - la figure 1 représente" sChématiquement une ligne de caractères en cours d'impression par une première tête d'écriture,
• - la figure 2 représente un premier diagramme temporel du courant de commande, de la température et de l'écriture associée d'un élément chauffant de la tête de la figure 1,
• - la figure 3 représente un deuxième diagramme temporel du type de celui de la figure 2,
• - la figure 4 représente six diagrammes temporels de six courants de commande, et les six écritures associées d'un élément chauffant de la tête de la figure 1,
• - la figure 5 représente un troisième diagramme temporel du type de celui de la figure 2,
• - la figure 6 représente schématiquement une ligne de caractères en cours d'impression par une deuxième tête d'écriture,
• - la figure 7 représente la configuration d'un "point prolongé décalé" imprimable par la tête de la figure 6,
• - la figure 8 représente un schéma par blocs d'un dispositif de commande de la tête de la figure 1,
• - la figure 9 représente un diagramme temporel de signaux d'entrée et de sortie du générateur de séquences du dispositif de la figure 8,
• - la figure 10 représente un schéma détaillé du générateur de séquences du dispositif de la figure 8, et,
• - la figure 11 représente un schéma détaillé du circuit retardateur du dispositif de la figure 8.

The invention also relates to a device for implementing the method of the invention, comprising

• means for driving said head in longitudinal displacement along a line to be printed, at a determined speed,
• means for heating said heating elements,
• - means for calculating the duration of the controlling heating said heating means, for subjecting said heating elements to thermal cycles each comprising an initial heating time and a cooling time, for printing dots,
  device characterized by the fact that
• the calculation means comprise means for adjusting the duration of the cycles to a duration at most equal to the duration of printing of the dots, means for determining certain heating elements to be subjected to heating from the middle of the cycles, and are arranged to control said heating means to subject said determined heating elements to heating for a period starting in the middle of the cycles.
  The invention will be better understood with the aid of the following description of the preferred implementation of the method, and of the preferred embodiment of the device of the invention, made with reference to the appended drawings, in which,
• FIG. 1 represents "schematically a line of characters being printed by a first writing head,
• FIG. 2 represents a first time diagram of the control current, of the temperature and of the associated writing of a heating element of the head of FIG. 1,
• FIG. 3 represents a second time diagram of the type of that of FIG. 2,
• FIG. 4 represents six time diagrams of six control currents, and the six associated entries of a heating element of the head of FIG. 1,
• - Figure 5 shows a third time diagram of the type of that of FIG. 2,
• FIG. 6 schematically represents a line of characters being printed by a second writing head,
• FIG. 7 represents the configuration of an "offset extended point" printable by the head of FIG. 6,
• FIG. 8 represents a block diagram of a device for controlling the head of FIG. 1,
• FIG. 9 represents a time diagram of input and output signals of the sequence generator of the device of FIG. 8,
• FIG. 10 represents a detailed diagram of the sequence generator of the device of FIG. 8, and,
• FIG. 11 represents a detailed diagram of the delay circuit of the device of FIG. 8.

With reference to FIG. 1, in a data transmission printer, for example, to print a line of characters 10 using a thermal print head 14 of serial type, the head 14 is moved in longitudinal displacement , here horizontal, along the line to be printed 10, in the direction of arrow 15, using a drive device which is not shown because it is conventional.

Line 10 is broken down into a series of transverse sections 10a, 10b, ..., 10m, ... whose horizontal dimension is equal to the horizontal dimension, or length, _L of an elementary point 100 that each of the heating elements 140 when it heats. The heating elements 140 are distributed along a transverse bar, here vertical, the height of which corresponds so at the height of the laundry 10.

Thus, each section 10m is printed during a writing cycle the duration T of which is equal to the length L of a point 100, divided by the drive speed of the head 14.

Each of the heating elements 140 is here a resistive element, accessible individually by connections not shown in FIG. 1, for the sake of simplicity. We will now describe the method of controlling one of these heating elements 140, the task of which is obviously to print a series of dots whose configuration depends on the line to be printed.

With reference to FIG. 2, each writing cycle of duration T is divided into two half-cycles of duration T / 2. It is expected to be able to heat, at the start of each half-cycle, the heating element considered for a duration TI for each first half-cycle and for a duration TID for each second half-cycle.

The heating element is heated here by passing it through a current I so that, taking into account its thermal inertia, after heating for the duration TI or for the duration TID, its temperature 0 remains above a temperature of threshold 6 _s for a time equal to T. As the threshold temperature θ _s is that above which the paper is printed, the point printed by the heating element heated for a period TI or TID corresponds to the length L.

• a) aucun chauffage :
  • pas d'impression
• b) chauffage pendant le premier demi-cycle seulement : impression d'un point normal de longueur L
• c) chauffage pendant le deuxième demi-cycle seulement : impression d'un point décalé de longueur L
• d) chauffage pendant les deux demi-cycles : impression d'un point prolongé de longueur légèrement supérieure à 1,5L.

As shown in Figure 2, there are four control possibilities for each cycle. These four possibilities are:

• a) no heating:
  • no printing
• b) heating during the first half cycle only: printing of a normal point of length L
• c) heating during the second half cycle only: printing of an offset point of length L
• d) heating during the two half-cycles: printing of an extended point of length slightly greater than 1.5L.

Indeed, in the latter case d) the fact that the heating element is heated during the first half-cycle results in the fact that the temperature reached during the second half-cycle, at the end of the time TID is greater than that reached at the same time in case c), for example. This results on the one hand in a risk of seeing the temperature exceed the maximum value θ _max admissible by the heating element, and on the other hand, a dragging of the writing of the extended point, which will be a little longer than 1.5L.

Such a drawback is encountered in other situations, and, in particular when writing an unbroken series of normal points, as shown in FIG. 3. In this figure, the fact that the temperature 6 of the element heating has not yet returned to room temperature A at the end of a writing cycle during which a dot has actually been printed results in a cumulative increase in temperature from cycle to cycle and an unacceptable temperature overshoot maximum θ _max ·

To overcome this drawback, the TI and TID durations of the heating phases are modulated so as to shorten them if the heated element is still hot at the start of each of these phases. Thus, the duration TI of heating during a first half-cycle takes the value T1 if the heating element has not been heated during any of the two half-cycles preceding this first half cycle; otherwise, the duration TI takes the value T2, with:

Ici, on choisit

Likewise, the duration TID of heating during a second half-cycle takes the value TID if the heating element has not been heated during any of the three half-cycles preceding this second half-cycle; otherwise, the duration TID takes the value T2D, with:

Here we choose

FIG. 4 shows, by way of example, a certain number of configurations to be printed and the shape of the heating current I.

FIG. 5 shows the result obtained in the case analogous to that of FIG. 3, of the writing of an uninterrupted series of normal points. It shows that the temperature remains below the maximum temperature θ _max .

Of course, the writing head 14 being provided with a plurality of heating elements 140 distributed on a vertical bar, at each first half-cycle of a cycle, a first set of points is heated which corresponds to the configuration of the first half-section of the section to be written corresponding to the cycle considered, and each second half-cycle, a second set which corresponds to the configuration of the second half-section.

avec k entier naturel et D mesurée entre les axes de chacune des deux colonnes.In the implementation of the process of the invention which has just been described, the thermal stresses on the elements heating elements are quite severe, in particular when writing a continuous horizontal line where the temperature 6 must remain permanently above the writing threshold θ _s . As a result, the life of the heating elements, and therefore of the head, is relatively short. To avoid this drawback, it is possible, in a known manner, and as shown in FIG. 6, to use a head 14 'provided with two transverse bars 141 and 142, arranged one after the other in the longitudinal direction, and distant from a length D equal to an even number of times the length L:

with k natural number and D measured between the axes of each of the two columns.

In this case, the first strip 141 is active during write cycles of odd order, while it is at rest during write cycles of even order. On the other hand, the second strip 142 is active during write cycles of even order, and at rest during write cycles of odd order. That is to say that the first strip 141 prints only the odd-order sections, while the second strip 142 prints only the even-order sections. Naturally, as shown in FIG. 6, the writing is only complete after the passage of the two bars.

As a result, each writing cycle during which an element has heated is necessarily followed by a writing cycle during which it can cool. The thermal stresses are therefore reduced and the service life increased.

Furthermore, considering a determined slice, if we order the bar that prints this slice to print an offset point, and the other bar to print a normal point at the same height as this point offset on the slice next, a fifth printing possibility f) is obtained, which is the "extended offset point" represented in FIG. 7.

Taking into account the very principle of thermal printing, the part common to the offset point and to the normal point does not present any difference in shade because all of the ink of the heat-sensitive paper, in the case of direct printing, or of the ribbon stationary relative to the paper, in the case of transfer printing, is released by the first heating element.

For the implementation of the method of the invention, the device shown in FIG. 8 is used, which receives on a bus 161 the data to be printed in the form of a digital signal and which controls the resistive elements 140 of the head 14 , including here a single bar of J elements.

Referring therefore to FIG. 8, a clock 1 of known type delivers on three outputs three clock signals SYP, SYD and H.

A sequence generator 2 receives the three signals SYP, SYD and H on three binary inputs and two digital signals, via two buses 21 and 22, on two digital inputs. It delivers on four binary outputs four sequential signals T1, T1D, T2 and T2D.

A decoder circuit 16 receives the coded data in digital form, via the bus 161, on a digital input, as well as the signal SYP on a binary input. It delivers two groups of J binary output signals on two buses 31 and 32 of type parallel to J bits, that is to say each comprising J parallel conductors.

A delay circuit 3 receives the signal SYP on a binary input and is connected to buses 31 and 32. The delay circuit 4 delivers three groups of J binary output signals tie on three buses 41, 42 and 43, of type parallel to J bits.

A multiplexer 4, of known type, of which a digital control input is connected to the digital output of a counter 6 receives the signals from the three buses 41, 42 and 43, and delivers three binary output signals on three outputs 71, 72 and 73.

The counter 6 is a modulo J counter, of known type, whose clock input receives the signal H and whose reset input receives the output signal from a detector 5 of the rising edges of the sequential signals T1, T1D, T2 and T2D, which it receives on four binary inputs. The counter 6 delivers a signal D on its overflow output.

A circuit 7 for calculating the information to be printed is provided with three binary inputs connected to outputs 71, 72 and 73, and four binary inputs receiving the sequential signals T1, T1D, T2 and T2D. It delivers, on two binary outputs, two INF and CE output signals.

A register 8, of the known type with serial input and parallel output at J bits, receives on a clock input, the signal H after passing through a logic gate 9 controlled by the signal D.

The J conductors of the parallel output of the register 8 are connected to a buffer register 10 with parallel input and output, controlled by the signal D after passing through a shaping circuit 11.

The J conductors of the parallel output of register 10 are connected to J AND gates 12 of which the J other inputs receive the signal CE.

The output of each AND gate 12 is connected to the control input of a controllable switch 13 disposed between a first terminal of a supply circuit 150 delivering the voltage VP and a terminal of one of the J resistive heating elements 140, the other terminal of which is connected to the second terminal of the supply circuit 150.

Naturally, the supply circuit 150 also supplies electrical energy to all the elements of the preceding circuits, in a manner not shown for the sake of simplicity.

Before describing in more detail the constitution of the blocks of the device of FIG. 8, let us approach in the operation.

Clock 1 delivers the signals SYP and SYD represented in FIG. 9, that is to say pulse trains with a frequency of recurrence equal to 1 / T and offset with respect to each other by T / 2. The signal H, not shown, is a train of pulses like the signals SYP and SYD, but of much higher recurrence frequency.

The sequential signals T1, T1D, T2 and T2D have the appearance shown in FIG. 9. The signals T1 and TID are at the high level for equal durations offset by T / 2, like the signals T2 and T2D. The duration of the sequence at the high level of the signal T2 is less than the duration of the sequence at the high level of the signal T1. The T1 and T2 signals go high during each pulse of the SYP signal. The TID and T2D signals go high on each pulse of the SYD signal.

The decoder circuit 16 develops, as a function of the data it receives on the bus 161, two groups of J binary signals which possibly change with each pulse of the signal SYP.

After the pulse of rank i of the signal SYP, the J binary signals of the first group are:

At the same time, the J binary signals of the second group are:

et

représentent deux bits de commande de l'élément chauffant 14, de rang j, pour l'impression de la tranche de rang i, conformément au code suivant

These two binary signals

and

represent two control bits of the heating element 14, of rank j, for printing the wafer of rank i, in accordance with the following code

sont transportés par le bus 31 et les J signaux du type

sont transportés par le bus 32.J type signals

are transported by bus 31 and the J signals of the type

are transported by bus 32.

,

, et

sur les trois bus 41, 42 et 43, respectivement.The delay circuit 3 delays these signals to deliver the three groups of J signals of the type

,

, and

on the three buses 41, 42 and 43, respectively.

,

,

, T1, T1D, T2, T2D, calcule le signal INF pour l'élément de rang j de la façon suivante :

After each rising edge of one of the sequential signals T1, T1D, T2 and T2D detected by the rising edge detector 5, the counter 6 is reset and controls, at the rate of the clock signal H the time multiplexing of the buses 41, 42 and 43 on binary outputs 71, 72 and 73. The circuit 7 for calculating the information to be printed calculates a binary signal INF which is at the high level if the heating element of rank j is to be heated and at the low level otherwise, for each of the four time intervals T2, (T1-T2), T2D, (T2D-T1D) of the range of rank (i - 1), so as to heat during time T1 or time T2, and during time TID or time T2D, depending, on the one hand, of the command for the range of rank (i - 1), and on the other hand of the state passed during the range of rank (i - 2) of this element, in accordance with the method described. The calculation circuit 7, from the seven signals applied to it,

,

,

, T1, T1D, T2, T2D, calculates the signal INF for the element of rank j as follows:

In a known manner, the J values of the signal INF calculated one after the other by the calculation circuit 7 are stored in the series-parallel register 8, after each rising edge of one of the sequential signals T1, T1D, T2 and T2D , then transferred to the buffer register 10 which controls, via the AND gates 12, the heating of the heating elements 14. The AND gates cannot be turned on if the CE signal is at the high level. This signal CE is produced by the calculation circuit 7 to be at the low level when the signal T1 and the signal T1D are normally at the low level, so as to ensure, even in the event of a malfunction of the circuits placed upstream of the doors 12, for example if T1 or TID remain at the high level at the end of the current half-cycle, periodic cooling of the heating elements to avoid destruction.

The current I which flows in each heating element 14 is here equal to the voltage VP divided by the value of the resistance of a heating element.

The constitution of the sequence generator is shown in FIG. 10. Two registers 23 and 24 receive the digital signals present on the buses 21 and 22 which, as will be seen below, control the durations of the signals _T 1 and TID of on the one hand and T2 and T2D on the other. The output of register 23 is applied to two gate circuits 25 controlled by the signals SYP and SYD. Similarly, the output of register 24 is applied to two gate circuits 25, controlled by the signals SYP and SYD. Four counters 26, each provided with a clock input receiving the signal H and a parallel input receiving the output of one of the gates 25 deliver, on their four sign outputs, the signals T1, T1D, T2 and T2D .

The operation of circuit 2 is as follows. Each counter 26 is mounted as a down-counter and its sign output is therefore positive, after having been loaded at the digital output value of the corresponding gate 25, only for a time which depends on this digital value. The signals shown in FIG. 9 are therefore obtained, the digital signal on the bus 21 controlling the duration, equal, of the signals T1 and TID and the digital signal on the bus 22 controlling the duration, equal, of the signals T2 and T2D. These durations can therefore be adjusted by the operator to adapt the device to the characteristics of the heating elements of the writing head, by modifying the digital signals applied to the buses 21 and 22.

et

correspondant aux différents caractères alphanumériques à écrire, dont la qualité d'impression sera évidemment meilleure, compte tenu des possibilités particulières au procédé de l'invention d'imprimer un point décalé et un point prolongé. Cette mémoire est adressée par des circuits d'adressage de type connu, en réponse aux données codées reçues sur le bus 161. Le circuit décodeur est donc de type connu, à la portée de l'homme de métier et ne sera pas davantage décrit.The decoder circuit 16 includes a memory which contains the different bit sequences

and

corresponding to the various alphanumeric characters to be written, the print quality of which will obviously be better, taking into account the particular possibilities of the process of the invention of printing an offset point and an extended point. This memory is addressed by addressing circuits of known type, in response to the coded data received on the bus 161. The decoder circuit is therefore of known type, within the reach of those skilled in the art and will not be described further.

The delay circuit 3 is shown in FIG. 11. It comprises on the one hand three buffer registers 34, 35 and 36 connected in cascade between the bus 31 and the bus 42 and on the other hand two buffer registers 37 and 38 connected in cascade between bus 32 and bus 43. A shaping circuit 330, the input of which receives the signal SYP, delivers at the output a signal which controls, on the one hand, a circuit of doors 333 interposed upstream of the register 36, and on the other hand, and after passing through a delay line 331, two gate circuits 332 each interposed upstream of the registers 35 and 38. The delay of the delay line 331 corresponds to the time necessary to transfer the data from one register like register 34 to the next like register 35, via a circuit of doors 332. The bus 41 is connected in bypass to the output of register 35, upstream of door 333. The operation of a such a circuit is obvious to those skilled in the art.

The rising edge detector 5, as well as the circuit 7 for calculating the information to be printed, are entirely conventional combinational logic circuits, the design of which is within the reach of those skilled in the art, and will not be further described.

Naturally, in case a head provided with two transverse bars is used, each of the bars is controlled by a device identical to that of FIG. 8, except that a single decoder 16, common to the two devices, is used .

et

de commande vers chacun des dispositifs.It is within the reach of those skilled in the art to design _the coupling of these two devices, by means of a common clock, controlling on the one hand each of the clocks 1 of each of the devices, and on the other hand a routing circuit disposed at the output of the common decoder 16 for alternately routing the bits

and

to each of the devices.

In the description which has just been made, the heating current I of the resistive elements is constant and its duration of application variable. In addition, the durations of the sequences at the high level of the signals T1 and TID on the one hand, T2 and T2D on the other hand, are equal. Furthermore, these durations are less than the duration T / 2 of a half-cycle. These characteristics, which allow relatively simple control, and a reduction in thermal stresses on the heads, are not compulsory.

One can in particular modulate the heating by varying the current I rather than the duration of its application, or alternatively by varying the two. Likewise, the durations of the sequences at the high level of the signals T1, T1D, T2 and T2D can be chosen as desired, while remaining less than the duration T of a cycle, in the cases where the head can withstand the resulting thermal stresses and where we don't try to get points of constant length.

Finally, the heating system by current flow in a resistive element can be replaced by any other heating system, for example by radiation, which may be suitable for thermal printing.

Claims (10)

1.- A method for controlling a thermal writing head (14) of the serial type for a printing system, provided with at least one transverse bar of heating elements (140), in which: - said head is driven in longitudinal displacement along a line to be printed (10), at a determined speed, the heating elements (140) being subjected to thermal cycles each comprising an initial heating time (TI-) and a cooling time for printing dots,
characterized by the fact - that the duration (T) of the cycles is chosen at most equal to the duration of printing the dots, - and that some of the heating elements are subjected to heating for a period (TID) starting in the middle of the cycles. 2.- Method according to claim 1, wherein, when a heating element is subjected to an initial heating for a period (TI) not zero, it can be subjected, within the same cycle, to a second heating. 3.- Method according to one of claims 1 and 2, wherein, if the initial heating time of a heating element is zero, within a cycle, it may not be subjected to heating that 'from the middle of the cycle. 4.- Method according to one of claims 1 to 3, wherein the duration (TI) of initial heating and the duration (TID) of heating starting in the middle of the cycle are less than half the duration of a cycle. 5.- Method according to one of claims 1 to 4, in which the duration (TI) of the heating is modulated, during a first half-cycle, of a determined heating element so as to shorten this duration if this heating element has has been heated during one of the two preceding half-cycles, and the duration (TID) of the heating, during a second half-cycle, of a determined heating element is modulated so as to shorten this duration if this heating element has been heated during one of the three preceding half-cycles. 6.- Method according to one of claims 1 to 5, wherein, the writing head (14 '? Being provided with a first bar (141) and a second transverse bar (142) arranged one after the other in the longitudinal direction, the elements of the second strip (142) are not subjected to any heating during an odd-order cycle, and the elements of the first strip (141 are not subjected to any heating) ) during an even order cycle. 7.- Device for implementing the method according to one of claims 1 to 6, for controlling a thermal writing head (14) of serial type for printer, provided with at least one transverse bar of heating elements (140) comprising: - means for driving said head (14) in longitudinal displacement along a printing line (10) at a determined speed, - heating means (8-13) of said heating elements, means of calculating (1-7, 16) the duration of the heating, controlling said heating means (8-13) to subject said heating elements (140) to thermal cycles each comprising a duration (TI) of initial heating and cooling time, to print points, device characterized by the fact that - the calculation means (1-7,16) comprise means (1) for adjusting the duration of the cycles to a duration at most equal to the duration of printing of the dots, means (16) for determining certain heating elements in front be subjected to heating from the middle of the cycles, and are arranged to control said heating means (8-13) to subject said determined heating elements to heating for a period (TID) starting in the middle of the cycles. 80- Device according to claim 7, wherein the calculation means (1-7,16) comprise means (7) for modulating the duration (TI) of the heating, during a first half-cycle, of a determined heating element so as to shorten this duration if this heating element has been heated during one of the two preceding half-cycles, and the duration (TID) of the heating, during a second half-cycle, of a determined heating element is modulated by so as to shorten this duration if this heating element has been heated during one of the three preceding half-cycles. 9.- Device according to one of claims 7 and 8, in which the means for calculating (1-7,16) the duration of the heating comprise: - a clock (1) of period (T) at most equal to the duration of printing of the dots, - a sequence generator (2), triggered by said clock (1) to generate at least one sequential signal (T1, T2) at the high level during the duration of the initial heating, and at least one sequential signal (T1D, T2D) at the high level during the heating period from the middle of the cycles, - a decoder (16) for determining, on the one hand, the heating elements to be subjected to the initial heating and, on the other hand, the heating elements to be subjected heating from the middle of the cycles, - a logic circuit (7) connected to said sequence generator (2) and said decoder (16) for controlling said heating means (8-13). 10.- Device according to claim 9, wherein - the sequence generator (2) generates at least two sequential signals (T1, T2) at the high level during the duration of the initial heating, and at least two sequential signals (T1D, T2D) at the high level during the duration of the heating from from the middle of the cycles, a delay circuit (3) with several outputs is interposed between said decoder (16) and said logic circuit (7), - said logic circuit (7) is connected to said outputs of said retarder circuit (3) and is arranged to modulate the duration (TI) of the heating, during a first half-cycle, of a heating element determined so as to shorten this duration if this heating element was heated during one of the two preceding half-cycles, and the duration (TID) of the heating, during a second half-cycle, of a determined heating element is modulated so as to shorten this duration if this heating element has been heated in one of the previous three half cycles.

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