EP0432024B1 - System and method of contrast control for a thermal printhead - Google Patents

Abstract

The subject of the invention is a method of contrast control for a print medium produced by a thermal print head (1) of a printer of a data transmission system comprising a series of resistive elements arranged to be activated by the passage of a current provided by a card (3) linked to a source of power (4) and in order thus to heat. The activation times of the elements are adjusted at the start of printing for a print cycle of minimal duration. For longer cycles, the elements are reactivated (14) as soon as their temperature while falling reaches the initial temperature, by a series of holding microcycles with very short durations of activation. If the printer of a fax machine is concerned, to which the invention applies particularly well, contrast can thus be adjusted as a function of the nature of the quality of the thermosensitive paper and of the rendering which it is desired to obtain with respect to an original document. <IMAGE>

Description

The present invention relates to a writing or printing head, thermal, for printer of data transmission systems, in particular of fax machines.

A thermal writing head comprises a plurality of resistive elements arranged to be traversed by a current, and therefore to heat, and to cooperate either directly with a printing medium, heat-sensitive paper, or indirectly with an ordinary printing medium, 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.

It can be a series type thermal printing, and then the movable head comprises at least one bar, generally vertical, of elements, aligned in a direction orthogonal to the direction of rectilinear displacement, during printing d '' a line of characters, corresponding to the height of the bar, of a carriage supporting the head.

It can also be a line type thermal printing and, in this case, the head, stationary, comprises a generally horizontal row of numerous elements, of width compatible with the width of the support intended to be printed during its scroll orthogonally to the row of elements.

From a so-called printer card, or from a transition card, the printing heating elements are supplied with current during determined time intervals, or activation times T _ON .

The contrast of a print medium is defined by the optical density, that is to say of surface area, of black. For thermosensitive paper, generally used in fax machines, this optical density, which is a function of the energy supplied to the heating elements, becomes saturated with a determined saturation energy. In the fax machines currently available, the heating energy of the resistive printing elements is preset for a given paper and to reach the saturation zone of the optical density of this paper.

In practice, fax machine users do not always use the paper for which these fax machines have been set at the factory. However, different papers do not have the same transition threshold, that is to say the same temperature from which the writing points begin to appear, so that the same heating energy does not allow 'reach all the respective saturation zones of these different papers, it being understood that the heating energy must not deteriorate the head or even reduce its service life. Still in practice, the same user may have to change paper. There is therefore a problem of maintaining the optimal contrast produced by the same thermal writing head on papers with different transition thresholds.

We already know, from document JP-A-63102958, a method for controlling the contrast of a print medium produced by a thermal writing head of the line type of a printer, of a data transmission system , comprising a series of resistive elements arranged to be activated by passing a current supplied by a card connected to a power source and thereby heating, in which by means accessible on the system, the times are controlled activation of resistive elements that can be varied.

According to this prior art control method, an operator can therefore, from a desk, adjust the contrast of a document to be printed, not only according to the nature or quality of this document but also , if it is a fax machine, depending on the rendering it wants to obtain compared to an original document, by operating the machine locally.

The energy applied to a heating element being equal to the product of the power supplied, fixed by the activation time T _ON , we can vary this energy from one document to another, whose transition thresholds are respectively different , to, in both cases, reach the saturation zone of the optical density.

But during printing, the running speed of the print medium may vary. In steady state, this running speed is constant and, in the case of a line type thermal writing head, the printing cycles of the heating elements have a constant duration and it can be assumed that the temperatures of the elements at the start and at the end of the cycles are almost equal. Under established conditions, and for a given printing medium, the printing energy is therefore the same from one cycle to the next. If the movement speed varies, the duration of the printing cycles varies.

The present invention aims to solve the problem of overcoming the variation of the duration of the printing cycles and of keeping the temperatures of the elements equal at the start and at the end of printing cycles, that is to say to keep the energy constant printing. In other words, the present invention aims to prevent the printing energy supplied by a heating element, during a printing cycle, and the contrast with it, from being all the more weak as the cycle previous is long.

To this end, the present invention relates to a method of the type defined above, characterized in that the activation times of the resistive elements are set at the start of printing for a printing cycle of minimum duration and, for printing cycles of duration greater than this minimum duration, the resistive elements are reactivated as soon as their temperature by lowering reaches their initial temperature at the start of the cycle.

Preferably, the resistive elements are reactivated until the start of the next printing cycle during a series of holding microcycles with very short activation times.

Thus, the temperatures of the resistive elements at the start and at the end of the printing cycle are maintained equal, or almost, whatever the duration of the printing cycle. The variation in the duration of the printing cycles therefore does not cause the printing energy to vary. The contrast value is kept constant from one line to another.

Naturally, the duration of the holding microcycles and the duration of activation during each of these microcycles are chosen so that the temperature of the elements at the end of the last holding microcycle is as close as possible to the initial temperature at the start of the cycle. printing.

The activation times of the resistive elements can be varied step by step and increased or decreased from a nominal time interval.

To increase the contrast, the activation time is increased and to decrease the contrast, the activation time is decreased.

The transmission system can be implemented so that these activation times are in fact controlled by the temperature of the writing head determined by a thermistor which controls these activation times and therefore the energy applied to the heating elements. The higher the ambient temperature, the lower the energy to be applied. During printing, the temperature around the head increases and tends to raise the temperature limits of the heating elements at the end of their heating phase and at the end of their cooling phase, i.e. upper limit temperature, or high point, at the end of the activation times and the lower limit temperature, or low point, at the end of the printing cycles. In other words, a drift tends to be created. In order to maintain a satisfactory impression and also to avoid overheating the resistive elements, this temperature drift is controlled, using the thermistor, to limit it to the minimum. As soon as the variation of these high and low points, relative to nominal levels, exceeds a maximum value predetermined in the factory, the activation times are reduced so that these upper and lower limits are lowered to their nominal values.

Thus, in the contrast control method of the invention, the transmission system comprising a head temperature control loop, set on nominal limit temperatures, to automatically control the activation times of the resistive elements as a function of the heating of the head, when the activation times are varied, the nominal limit temperatures can be varied.

The operator can therefore adapt the printing, certainly to the desired contrast, but above all to the printing medium used and to its transition threshold. It will be emphasized here that an activation time is therefore the sum of a time determined automatically by the control loop and a differential time entered manually by the operator.

Preferably, the nominal limit temperatures can only be varied within ranges predetermined at the factory.

The present invention also relates to a data transmission system comprising a printer with a line type thermal writing head for printing a print medium, the contrast of which is controlled according to the method of the invention, the head comprising a series of resistive elements activated from a printer card connected to a power source, system characterized in that it includes a console provided with keys for varying the activation times of the resistive elements, means for adjusting the times at the start of printing activating the resistive elements for a printing cycle of minimum duration and means for reactivating the resistive elements during printing cycles of duration greater than this minimum duration.

• la figure 1 représente une vue schématique d'une imprimante d'un télécopieur mettant en oeuvre le procédé de contrôle de contraste de l'invention;
• la figure 2 représente deux courbes de variation de la température d'un élément chauffant de la tête de l'imprimante de la figure 1, pour deux papiers thermosensibles à seuils de transition différents;
• la figure 3 représente la courbe de variation de la température d'un élément chauffant de la tête de l'imprimante de la figure 1, sur plusieurs cycles d'impression, régulée par une boucle d'asservissement;
• la figure 4 représente un organigramme de la combinaison des contrôles automatique et manuel des temps d'activation et
• la figure 5 représente la courbe de variation de la température d'un élément chauffant de la tête de l'imprimante de la figure 1, soumis à des microcycles de réactivation pendant un long cycle d'impression avec une phase de refroidissement prolongée.

The invention will be better understood with the aid of the following description and explanations, with reference to the appended drawings in which

• FIG. 1 represents a schematic view of a printer of a facsimile machine implementing the contrast control method of the invention;
• FIG. 2 represents two curves of variation of the temperature of a heating element of the head of the printer of FIG. 1, for two heat-sensitive papers with different transition thresholds;
• FIG. 3 represents the curve of variation of the temperature of a heating element of the head of the printer of FIG. 1, over several printing cycles, regulated by a control loop;
• FIG. 4 represents a flowchart of the combination of the automatic and manual controls of the activation times and
• FIG. 5 represents the curve of variation of the temperature of a heating element of the head of the printer of FIG. 1, subjected to microcycles of reactivation during a long printing cycle with an extended cooling phase.

With reference to FIG. 1, the thermal writing head 1 is that of a printer of a facsimile machine using, as printing medium, heat-sensitive paper whose contrast produced by the head is controlled. The head is of line type and comprises a row of resistive elements arranged in line orthogonally to the direction of travel of the paper on a drum 2. The elements of head 1 are activated by passing a supplied current by a printer card 3 from a power source 4. The card 3 essentially comprises a battery of switching transistors turned on during activation times T _ON by an activation circuit 5. The facsimile machine is naturally provided with 'A microprocessor 6 and a desk 7 for controlling activation times. The activation times T _ON are set at the very start of printing and the console 7 allows, using an increase and a decrease key, to vary the value of these values step by step. activation time, by controlling an up-down counter 8 connected, at input, to a clock generator 9 and, at output, to microprocessor 6 which controls the activation circuit 5 to generate a differential variation of times d 'activation.

The writing head 1 is inserted into a temperature control loop 10. A thermistor 11 captures the temperature of the head 1. A memory 12 contains nominal upper and lower limit temperature values and the actual upper and lower temperatures are compared with the nominal values in a comparator 15 which regulates the activation times provided by the circuit 5. The control loop 10 is set to the nominal limit values and automatically controls the activation times T _ON as a function of the heating of the head. Desk 7 allows an operator to manually control these activation times by adding differential times, positive or negative, depending on the desired contrast. As explained below, the manual modification of the activation times modifies the nominal limit values of memory 12.

The duration of the printing cycles, which can vary during printing, is measured by a measuring chain 13, downstream of the drum 2, in order to be able, in the event of printing cycles of duration greater than a minimum duration, reactivating the resistive elements of the head 1 via a secondary activation circuit 14 also controlling the printer card 3.

The following explanations will allow a better understanding of the contrast control process.

Either a heating element of the writing head 1 to which an energy equal to the product of the power, supplied by the source 4 and the card 3, is applied by an activation time T _ON (FIG. 2) . From the ambient temperature ϑ _o , the temperature of the element increases in time until time T _ON where it reaches the value ϑ _s , following a portion C _a of the curve representative of the variation of the temperature of the element as a function of time. Beyond the point (T _ON , ϑ _s ), the temperature decreases following a portion of curve C _d . Or a heat-sensitive paper with a transition threshold ϑ _t . The surface portion, in Figure 2, hatched at an angle of 45 degrees, between the curve portions C _a , C _d and the ordinate line ϑ _t , is representative of the writing energy on the paper with transition threshold ϑ _t . Let us admit that it corresponds to the desired contrast. Now consider a paper with a transition threshold ϑ _t ′ greater than ϑ _t . The surface portion, between the curve portions C _a , C _d and the line of ordinate ϑ _t ′, doubly hatched, is less than the first surface portion defined above. It represents insufficient writing energy to produce the desired contrast. In this case, the operator presses the "increase" key on the desk 7 to increase the activation time by one or more steps and raise it to T ′ _ON , greater than T _ON . The temperature of the heating element reaches at the instant T ′ _ON the value ϑ ′ _s , greater than ϑ _s then descends along a portion of curve C ′ _d . The surface portion, comprised between the portions of curve C _a , C ′ _d and the straight line of ordinate ϑ ′ _t , hatched at an angle of 135 degrees, is substantially equal to the first and represents a writing energy producing the desired contrast.

x représentant le nombre d'actionnements des touches.The action of the operator on the keys of the desk 7 makes it possible to vary, by step ΔT _ON , the activation times, from a preset value T _ON , to increase or decrease the contrast on a determined paper, according to relationships

x representing the number of key operations.

FIG. 3 represents, still as a function of time, the variation of the temperature of a heating element of the head 1 during several printing cycles, corresponding to several writing lines, but regulated, after the first cycle d impression represented by the servo loop 10. The loop 10 makes it possible to interrupt the drift of the upper and lower temperatures of the heating elements when these limit temperatures exceed by a predetermined value the nominal limit temperatures ϑ _ls and ϑ _li . The element activation time during the first cycle shown is T _ON . At the end of the activation time T _ON , the upper temperature ϑ _s exceeds ϑ _ls by the maximum permitted value, after having drifted along curve D.

pour rabaisser, à la fin de ce nouveau temps d'activation, ϑ_s à ϑ_ls avant que ϑ_s ne dérive à nouveau dans le temps le long de la courbe D′ et ainsi de suite. La température inférieure ϑ_i est naturellement soumise à la même régulation.Loop 10, in the next cycle, reduces the activation time to T ${}_{\text{O}\overline{\text{NOT}}}$

to lower, at the end of this new activation time, ϑ _s to ϑ _ls before ϑ _s drifts again in time along the curve D ′ and so on. The lower temperature ϑ _i is naturally subject to the same regulation.

In fact, the automatic control provided by the control loop 10 and the manual control performed by the operator combine. The implementation of one or other of the relationships I above causes a displacement of the nominal limit temperatures within ranges, high and low, predetermined in the factory.

The combination of these two controls is illustrated by the flowchart in FIG. 4. The temperature O of the head 1 is entered by the thermistor 11 which regulates the value of the activation time T _ON . If the operator is satisfied with the contrast, T _ON is maintained and printing is carried out with T _ON . If the operator wants to increase the contrast, for example by ΔT _ON , printing is carried out with the activation time T _ON + ΔT _ON before the thermistor re-enters the temperature of head 1.

With reference to FIG. 5, which also represents variations in the temperature of a heating element as a function of time, the curve C extends over a first relatively long cycle at the end of which the lower temperature ϑ ′ _i of the element is lower than its starting temperature ϑ _i , so that, during the following cycle, the higher temperature ϑ _s ′, at the end of the activation time, is lower than the temperature ϑ _s of the previous cycle and n ' is not sufficient for the writing energy, for a paper with a transition threshold ϑ _t , to produce the desired contrast. In these conditions, the secondary activation circuit 14 reactivates the heating element as soon as its temperature, falling, reaches the temperature ϑ _i , at time T _r in FIG. 5, and until the start of the next cycle, at time T _d , during holding microcycles Γ with very short activation time. Under these conditions, the temperature curve of the heating element deviates, during the cycle following the long cycle, from curve C so, at the end of the activation period, again reaching the upper temperature ϑ _s appropriate producing the desired contrast.

We have seen that the ambient temperature of the medium in which the writing head 1 is located was controlled by the thermistor 11 and that, as a function of this temperature, a greater or lesser heating energy was applied to the resistive elements by adjusting the activation time. If we denote by E _n , the nominal energy at 25 degrees C, the heating energy to be applied can, for example, vary from 1.3 E _n , at 0 degrees C, to 0.8 E _n at 60 degrees C for a given head.

If we consider for example a printing cycle of the heating elements of a line type writing head equal to 10 ms, a duration T _ON ²⁵ of nominal activation, at 25 degrees C, of 0.8 ms and a maximum activation time T _ON ^max of 1.25 ms, at temperatures 0 degrees C and 60 degrees C, the heating elements must respectively be activated for

For a cycle of 5 ms, a duration T _ON ²⁵ 0.6 ms and a duration T _ON ^max 0.625 ms was

As a result, for a printing cycle of 5 ms, the system cannot function properly at a temperature below 25 degrees C, the energy likely to be applied to the heating elements then no longer being sufficient. Admittedly, these are infrequent situations because, if the temperature of the head is less than 25 degrees C, when the power is turned on, it quickly exceeds this temperature after printing a few documents and warming up the temperature. system electronics.

Anyway, and rather than oversizing the current supply to the system to be able to activate the elements for double durations, with the additional disadvantage of a more significant heating, when the temperature of the head, entered by thermistor 11 is less than a predetermined threshold and the duration of the printing cycles would be, at least temporarily, too short, here by 5 ms, the duration of the cycles is lengthened, here by doubling it to 10 ms, up to the ambient temperature around the head exceeds said threshold, beyond which the cycles are shortened and resume their nominal duration, here by 5 ms.

Claims (8)

1. Procedure to control the contrast of a printing material produced by a thermal writing head (1) of the line type of a printer of a data transmission system, comprising a series of resistive elements designed to be activated by passing a current supplied by a card (3) linked to a power source (4) and thus to heat, in which, using means (7) which are accessible on the system, the activation time of the resistive elements which can be made to vary is controlled, characterised by the fact that at the start of the print, the activation time of the resistive elements is adjusted for a print cycle with a minimum duration and, for print cycles with a duration greater than this minimum duration, the resistive elements are reactivated as soon as their falling temperature reaches the initial temperature at the start of the cycle.
2. Procedure according to claim 1, in which the resistive elements are reactivated until the start of the cycle following the print during a series of maintenance microcycles of very short activation durations.
3. Procedure according to one of claims 1 and 2, in which the activation time can be made to vary step by step.
4. Procedure according to one of claims 1 to 3 in which the activation time can be increased and reduced based on a nominal time.
5. Procedure according to one of claims 1 to 4 in which the system comprises a temperature head (1) servo-control loop (10), keyed onto one of the nominal limit temperatures to control automatically the activation time of the resistive elements according to the heating of the head, when the activation times are made to vary, the said nominal limit temperatures are varied.
6. Procedure according to claim 5, in which the said nominal limit temperatures are varied within predetermined bands.
7. Procedure according to one of claims 1 to 6 in which, when the head (1) temperature is lower than a predetermined threshold and when the duration of print cycles are too short to apply a sufficient heating energy to the resistive element, the duration of the cycles are lengthened until the head temperature exceeds the said threshold.
8. Data transmission system comprising a line type thermal writing head printer to print printing material whose contrast is controlled according to the procedure in claim 1, the head (1) comprising a series of resistive elements activated from a print card (3) linked to a power source (4), system characterised by the fact that it comprises a desk (7) fitted with switches to vary the activation time of the resistive elements, means to adjust the activation time of the resistive elements at the start of printing for a print cycle with a minimum duration and means (14) to reactivate the resistive elements during print cycles with a duration greater than this minimum.

Images