FR2808476A1 - Method of driving a thermal print head, uses stored values on duration of excitation of thermal elements around print point after obtaining values by experiment - Google Patents

Abstract

The method of driving a print head uses experimental data to control duration of activation of thermal elements (26). A set of patterns (13) is proposed round a point, and elements in the pattern round the point activated for different durations. The resulting print is inspected and the pattern giving the sharpest image retained to control driving of elements for each point.

Description

A thermal print head, and here will be considered a "line head", comprises a row of individually activated spot elements for, pixel by pixel, transferring the ink of a printing film a sheet to be printed, or to directly print thermal paper. Some fax machines have such heads. As the printing of the successive lines is carried out at a high rate, which it excludes to reduce, the heating elements do not fall back to exactly the ambient temperature between two line prints.

In particular, the temperature at which a heating element drops after each activation of an uninterrupted succession of activations, to print a column of black spots, drifts progressively increasing. The two heating elements on either side of the element in question also contribute to this thermal drift, which causes an excessive transfer of ink, dilating the size of the printed pixels.

Conversely, a black pixel preceded by a column of white pixels may be printed as a point too small, a heating element too cold.

This problem of overflowing one pixel on the area of another also poses for color printers.

In an attempt to control the temperature of the heating elements, it is known to model the thermal exchanges of each of these, as a function of the pixel patterns that have been printed before in the column of the element in question and in the adjacent columns and in function. ambient conditions of individual heating and general cooling by ventilated radiator. Thus, if one determines, by calculation according to such a model, that a heating element has a falling temperature which drifts, the future activation is corrected. Such models of heat exchange are difficult to establish precisely and, even after a long development, they do not always give satisfaction, the isolated points, black and white, being poorly restored as soon as the operating conditions deviate from model.

The present invention aims to provide an alternative solution to the printing problem mentioned above.

The Applicant has indeed found that the above problem, control of the size of the printed pixels, only really arises in the cases of restitution of details, where the pixels of a type, black or white are in clear minority in front of those of the other type, such as an isolated white black pixel or a line representing a character but in video inversion.

In short, it was enough to deal only with the problem of controlling the heating elements located in detail image areas, which may represent such imbalanced configurations between pixels of different types, by modifying the controls in favor of the minority.

This is how the plaintiff proposes her invention.

To this end, the invention relates to a method for controlling a thermal printing line head comprising a plurality of heating elements for printing lines of image points, characterized in that - a database of pattern patterns around a point, - activation patterns are experimentally associated with the patterns, and for each image point to be printed by a heating element, - its image environment is compared with the pattern patterns to retain them. a and - order the activation of the element in question according to the duration associated with the chosen pattern. Thus, it is the content of the image area considered that directly determines the controls of the heating elements, without the need for a calculation to take into account the estimated temperature of those '. Advantageously, to create the pattern bank and associate the activation times thereof, - zones are determined in a test image to be printed, each containing a point to be printed, - patterns are memorized representing the zone points to form the bank. of patterns, - each pattern is associated with a heating time, fixed a priori, of a printing element of the point considered, - points are printed patterns according to the associated heating times and - by examination of defects of In print, the associated times are empirically corrected for, by a succession of impressions, examinations and corrections, to determine durations providing a desired level of print quality.

The invention will be better understood with the aid of the following description of a preferred embodiment of the method of the invention, with reference to the accompanying drawing, in which - FIG. 1 very schematically represents a line head of FIG. thermal printing and sheet being printed, - the figure shows a heating element of the head, its particular control logic circuits and common control circuits, and - Figure 3 shows six patterns of a pixel bank.

The thermal print line head with the reference numeral 1, FIG. 1, belongs here to a fax printer and prints the successive lines of a sheet of paper 30. A number of basic graphic patterns 31 such as letters and numbers have already been was printed, the sheet 30 being driven step by step, line by line, upwards in Figure 1, by a mechanism not shown, to scroll in front of the head 1. An ink ribbon, provided here for printing ordinary paper , has not been represented.

The head 1 here comprises a row of 1728 point heating elements for printing as many columns of black dots, for an A4 format '200 dots per inch. Reference 2 designates one of these heating elements and its control electronics, which are shown in detail in FIG. 2. The heating elements are here grouped into four successive blocks 5, 6, 7, 8 with separate controls, that is, blocks supplied separately from a + 24 V source through a switch transistor like the one referenced 32 and shown functionally in FIG. 2, only one of the four supply transistors 32 being in the closed state at a given instant .

In FIG. 2, the heating element carries the reference and is individually supplied with +24 V through an individual control switch transistor 25, here ground return. Transistor 25 is controlled by a stage 20 of a series / parallel shift register 28 which cyclically receives, every 10 ms, a new set of 1 write or line print control bits. A bit at logic level 1 here controls the conduction of transistor 25, hence the black dot printing, a bit at 0 being inactive.

The stage 20 comprises a memory point 21 connected, by an input 22, to the memory point of the upstream stage and connected at the output to a downstream stage and to a first input of an AND gate 24 for controlling the transistor 25. The activation control input 23 of the heating element 26 is connected to a second inhibition input of the door 24.

The shift register 28, comprising 1728 stages similar to the stage 20, supplied with image data from a memory 12 containing the electronic image of the page to be printed. A microprocessor 11, operating with a time base circuit 10, manages the memory 12 and the register 28, that is to say cyclically controls the writing of 1728 bits of line in it and also controls the control input 23 of the output port 24. An external link not shown is connected microprocessor 11 to provide the image data, it transferred the memory 12. To set the individual activation times of the various elements 26, activated, activatable here all at the same time by the same switch 32 for the same block 5 to 8, there are provided individual inhibition circuits of the outputs of the stages 20. For this purpose, a demultiplexer 29, integrated in the register 28, comprises, for the various stages 20, 1728 respective outputs addressable by the microprocessor 11. Each output of the demultiplexer 29 comprises a memory point not shown controlling the output gate 24, to trav to the input 23, to maintain the inhibition until the end of each cycle of 10 ms of writing a line. In a variant, the door 24 is omitted and the demultiplexer 29, without added memory point, controls the individual reset of the memory point 21 of the shift register 28. According to another variant, the shift register 28, with the demultiplexer 29 to addressable parallel output memory points, is replaced by this single demultiplexer 29 with addressable output memories (equivalent to the memory points 21 but with parallel inputs and outputs) directly controlling the various transistors 25. The multiplexer part itself provides an activation bit in 1 by the selected output, at the input (22) of the value bit of the memory point (21) of this output, and passes, only to this memory point (21), common signal (not shown) clock memorisation. In multiplexer, an additional input (not shown) for a value bit (deactivation) directed towards one of the memory points (21), or of global inhibition of the outputs of the multiplexer part itself, makes it possible to apply a 0 to the value input of the addressed memory point (21).

The activations and deactivations of the various heating elements 26 can thus, in various embodiments, intervene at any desired time. It can be provided a deactivation or activation common to all the memory points 21, controlled by an input link, with, in the second case, an individual deactivation almost immediate memory points wrongly activated.

A memory 13 containing a bank of image type patterns is further connected to the microprocessor 11.

The control of the various heating elements 26 of the head 1 will now be explained in more detail.

To control a heating element, such as element 26, for example, a bank of typical patterns is created around a point, precisely the point to be printed by the element, the bank being stored in memory 13, it is experimentally associated with patterns of the activation time bank and, for each image point to be printed by the heating element 26, in the pixel column that it processes and for the line to be printed, its image environment is compared with the typical patterns to retain one, the one that most resembles this environment, and - it controls the activation of the heating element considered in accordance with the duration associated with the type pattern retained.

The first two steps above are performed by an operator controlling the microprocessor 11, in particular to write in pattern memory 13, the last two steps being performed directly by the microprocessor <B> 11 </ B> itself.

FIG. 3 shows some of the standard patterns having an associated heating element activation time indication or note 26. Each pattern here is a nine-bit square, or pixels, with the heating element 26 being controlled. by the central, reference, (underlined) bit of the bottom line of the pattern. The two top lines have already been printed.

In the pattern 40, with three rows of bits 000, 000 and <B> <U> 010, </ U> </ B> the point to be printed is isolated in the middle, or under, white dots. The heating element 26 is therefore relatively cold and it is therefore necessary to associate a maximum activation time, noted on a scale of 1 to 5. It is possible in particular during a printing period of 10 ms line , perform, in addition to the initial loading, several reloads of the register 28, for example a total of 5 loads, to process the 5 notes. During the first of the reloads, the bit of each pixel of note 1 (minimum duration) is reset to 0, and so on, the only remaining in 1, until the end of the 10 ms, the note bits 5.

Pattern 41 (000, 000, 110) includes, in addition to pattern 40, a dot to be printed adjacent to the reference point of interest. The heating element 26 will thus be heated indirectly by the adjacent element and, moreover, is adjacent to two white pixels, from the right and from above and to two other white pixels at an angle, in which it is necessary to avoid overflowing, the activation time has the average rating 3.

The pattern 42 (000, 110, 010) may be considered to have the pattern 41 shifted upward by 1 line. In this case, the fact that the reference bit is between two 0 shows that it is a bend of a fine black line. We therefore associate note 4.

The pattern 43 (000, 100, 010) has an intermediate configuration between the patterns 40 and 41, in that the two bits at 1 appear to represent a thin line section diagonally. The activated heating element 26 will not be simultaneously heated indirectly by its neighbor. It is therefore note 4 which is retained.

Pattern 44 (000, 000, 111) has a lower horizontal black line to print and average score 3 is assigned.

The pattern 45 (000, 111, 111) has the pattern 44, then reassembled line in mid-height position. The print line of the pattern 45 being also black, considers a priori that it is the edge of a larger black block appears on the print, the pixel to print being this time in the middle of this black block emerging . We can then assign the note 5 to print a black dot possibly overlapping the adjacent black spots and thus form a perfectly black block without discontinuity between pixels.

To simplify the commands, the heating elements like the one referenced 26 are controlled here with activation times in a number smaller than that of the patterns and precisely, as indicated above, there are 5 possible activation times corresponding to the respective notes. ranging from 1 to and extending in this order substantially between 0.5 ms and 1 ms.

In practice, each pattern is represented in a one-to-one manner by a number representing the cumulative weighting values assigned, fixedly but arbitrarily, to the various pixel positions of the pattern, values following a geometric progression here of 2: 1, 2, 4, 8, etc., the only black pixels involved.

To create the pattern bank and associate the activation times with them, - zones are defined in a test image to be printed, each containing a point to be printed, - patterns are memorized, like those in FIG. the zone for constituting the pattern bank, - each pattern is associated with a heating time, fixed a priori, of a print element 26 of the point in question, - the patterns of the patterns are printed according to the associated heating times and, - by Examination of print defect, the associated durations in memory 13 are empirically corrected for, by sequence of printing, examinations and corrections, determining durations providing a desired level of print quality.

In this example, an additional activation control correction above is additionally made to take into account the average temperature of the head 1 in the zone of each heating element 26. The heating elements are, as indicated, grouped in blocks 5 to 8. with separate commands, here a series of four identical blocks which are controlled (32) successively to limit the maximum current. It thus has, from the common source, four power supplies + 24 volts, each through an electronic switch (32) closed, at most, a quarter of the time.

head 1 comprises a radiator thermally coupled to the heating elements as 26 and which carries at one end a radiator temperature sensor 3 connected to the microprocessor 11.

microprocessor 11 can thus accurately measure the temperature of a radiator section relative to the block closest to the sensor 3, but, on the other hand, the instantaneous temperature of the farthest section, relative to the block 8, is poorly determined; makes the thermal transit time from one end of the radiator to another, which is not negligible time.

To correct the effect of this delay in the measurement, - having measured the temperature of the head 1 at the sensor 3, - is calculated for each block 5 to 8 of heating elements 26, the energy supplied on a series determined by lines, for example the last 50 printed, - weights the measured temperature of the head 1 by the energy supplied per block considered 5 to 8, and - the activation times of the heating elements 26 are corrected by the weighted temperature of the head 1.

In other words, and in a practical manner, the set of activation times is replaced by a plurality of games of the same kind so as to be able to modulate each of the five durations, initially fixed, and thus adapt them generally to the local thermal conditions. from block 5 to 8. Since the temperature correction, and therefore the duration of activation, by the energy supplied is of limited amplitude, a satisfactory result is obtained by calculating the energy supplied by a simple accumulation of the number of dots printed in the series of lines that precede, regardless of their position (column) in the block 5 to 8 considered and even possibly regardless of their line, that is to say without image analysis. However, we can weight the thermal consideration of the various pixels already printed in inverse function of their distance to the line being printed and for example only weight by half the printed line pixels distant from more than 25 lines of the line. print course.

 Moreover, takes into account the position of the block considered 5 to 8 ratio to the temperature measurement point (3), to take into account the dispersions of the thermal transit delay of the various blocks 5 to 8 to the sensor 3, which varies as the distance of these to the sensor 3.

Claims (1)

 1. A method for controlling a head (1) thermal printing line comprising a plurality of heating elements (26) for printing lines of image points, characterized in that - one creates a bank of patterns (13) around a point, - the activation times are experimentally associated with the patterns and, for each image point to be printed by a heating element (26), its environment is compared with image with the standard patterns (3) to retain one and - control the activation of the considered element (26) according to the duration associated with the selected pattern. 2. A process according to claim 1, wherein the heating elements (26) are controlled with activation times in number less than that of the patterns. 3. A method according to one of claims 1 and 2, wherein, to create the pattern bank (13) and associate their activation times, - is determined in a test image to print, zones containing each point to be printed, - patterns representing the points of the zone are stored in order to constitute the pattern bank (13), - each pattern is associated with a heating duration, fixed a priori, of a printing element (26). from the point considered, - the points of the patterns are printed according to the associated heating times and, - by examination of printing defect, the associated durations are periodically corrected for, by a succession of printing, of examinations and corrections, determine durations providing a desired level of print quality. 4.- Method according to one of claims 1 to 3, wherein, heating elements (26) being grouped in separate control blocks - the temperature of the head is measured, - for each block (5) is calculated, energy supplied on a given series of lines, - the temperature of the head is weighted by the energy supplied by the block (5 8), and - the activation times of the elements (26) are corrected by the weighted temperature of the head. 5.- Method according to claim wherein, taking into account the position of the block considered (5-8) relative to the measuring point (3) of the temperature. 6. A method according to claim wherein weight is taken into account the various pixels already printed in inverse function of their distance to the line being printed.