EP0535769A1 - Method for the compensation of resistance tolerance in printing multi-tone images - Google Patents

Abstract

Known thermal print heads have heating elements whose resistors are subject to tolerance. The invention relates to a method for the compensation of resistance tolerance in printing multi-tone images. Setting out from a smaller number of required tone stages (TS1 to TS5) as available tone stages, represented by the energy values (E1 to E15), printing always takes place with the available tone stage whose available optical density (VOD) approximates to the required visual density (OD(TS)). The differences between required optical density (OD(TS)) and actually available optical density (VOD) are summated to obtain a mean-value deviation whose magnitude is minimised by selecting the corresponding available tone stage such that a visual image produced has the required optical density (OD(TS)). <IMAGE>

Description

The invention relates to a method for compensating the resistance tolerances when printing a multi-tone image with a printing device which has printing elements arranged in a row. The invention is particularly suitable for thermal printing recorders in which color particles are transferred to or into a call drawing medium from an ink ribbon.

In the case of a printing device with multi-tone reproduction, recording data are generally converted into printing pulses which have a pulse width corresponding to the tone levels for the supply to suitable heating resistors. This process controls the amount of toner transferred from a toner film to effect the recording of a desired density. The heating resistors are heated to selected temperatures by supplying pressure pulses based on the sound data. However, if there are differences in the resistance values between the heating resistors, the heating resistors are heated to different temperatures even if their pressure pulses are the same Pulse width can be supplied. As a result, the recorded image shows density irregularities, and thus a low quality recording.

However, such differences in the resistance value of the heating resistors appear to some extent as an inevitable side effect because it is difficult to make the resistance values uniform in the number of heating resistors arranged in a row. Under these circumstances, an average resistance value is given for all heating resistors of the thermal head.

It is also known that heating resistors deteriorate and resistance values increase over time. If no measures are provided in the thermal head or in the housing of the recording device to compensate for the deterioration of the heating resistors, the thermal head is no longer usable if the resistance value increases by approximately 10%.

From publication DE 38 20 927 a thermal pressure recording device with multi-tone reproduction is known which, in addition to a thermal head with a number of heating resistors arranged in a row, means for measuring the resistance of each individual heating element and a compensation memory for storing information for use in the compensation of sound data which indicate the tone levels of points to be recorded as a function of the resistance values of the heating resistors for recording the points. The information stored in the compensation memory is variable with the resistance values measured by the measuring device at selected times. The means for driving the heating resistors are suitable for reference to the information stored in the compensation memory.

The compensation memory can be designed to store compensation data, to determine the pulse width to be supplied to each of the heating resistors from the resistance value of each heating resistor and the tone levels indicated by the sound data. Although this multi-tone recording device is able to react immediately to all changes regarding the heating resistances of the thermal head, the device-related expenditure is very high and additional time is required to realize the resistance detection between the individual printing processes. This slows down the printing process as such, which is particularly undesirable at high resolutions.

The compensation principle implemented in the known multi-tone recording device is fundamentally designed to generate any optical density desired according to image data by means of analog value color transfer to the recording medium.

From the application DE 40 25 793.2 a method for printing a halftone image is known, in which a large number of level values are generated by reference pattern assignment from a small number of required tone levels.

The invention has for its object to compensate for any resistance tolerances of thermoelectric printing elements in a printing device for printing a multi-tone image with minimal ongoing technical expenditure.

According to the invention, this object is achieved in that, with a predetermined number of tone levels of required optical density and a larger number of physically available tone levels, each dot is printed with the pressure level whose resistance-specific associated available optical density is closest to the required optical density lies and the deviation of the available optical density from the required optical density is compensated for, including an immediately adjacent physically available tone level, so that the mean value of the actual optical density of successive pressure points corresponds to the required optical density.

In an embodiment of the invention, a number of successive resistance ranges is specified once so that the resolution, based on the nominal value of the resistance, reliably falls below the resolution of the human eye. The resistance value of each pressure element is measured and each pressure element is assigned to one of the resistance ranges. A value is assigned to each required and each available optical density. For each printing element, the difference between the values of the required optical density and the currently available optical density is defined as the mean value deviation.

Depending on the current value of the mean value deviation, the correspondingly available tone level is selected cyclically for each point to be printed and the mean value deviation for the following pressure point is determined.

In a further embodiment of the invention, the sum of the current mean value deviation and the value of the required optical density is formed and this sum is compared with the value of the required optical density.

In a further step, the available tone level is used for printing, the available optical density of which, given the smaller sum, just falls below the required optical density and otherwise just exceeds the required optical density.

In a further embodiment of the invention, printing is carried out with the available tone level, the available optical density of which is at most equal to the sum of the deviation from the mean value of the present printing step and the required optical density.

In a further embodiment of the invention, the mean value deviation for the following pressure point is determined from the sum minus the value of the available optical density with the associated tone level of which the current point is printed.

In summary, the essence of the invention is to make targeted use of the integration behavior of the human eye caused by inadequate resolution, in that for each desired, quasi-analogue tone level, an optical mean value is formed by targeted alternative activation of two successive available tone levels, which corresponds to the desired tone level. Advantageously, only a small number of quantized tone levels are required to carry out this method, the long-term reproducibility of which can be ensured by additional measures.
For this purpose, the pressure elements are preferably pre-aged by applying a predetermined number of pressure pulses before they are measured.

In addition, all measuring means and means for processing the measured values within the device can be dispensed with, as a result of which the expenditure on device technology is reduced.

Furthermore, the printing process is not interrupted by measuring cycles and thus a higher throughput of recording material per unit of time is achieved.

When changing the tone level within a line to be printed by a printing element, the value of the mean value deviation is advantageously adopted in order to achieve the desired edge contrast.

Fig. 1

eine Darstellung des Zusammenhanges der optischen Dichte mit der zugeführten Energie in Abhängigkeit vom Widerstand des Heizelementes und

Fig. 2

eine Darstellung des Zusammenhanges verfügbarer Tonstufe zu verlangter Tonstufe in Abhängigkeit von der optischen Dichte und den maximalen Widerstandstoleranzen.

An embodiment of the invention is shown in the drawings and is described below. It shows:

Fig. 1

a representation of the relationship of the optical density with the energy supplied as a function of the resistance of the heating element and

Fig. 2

a representation of the relationship of available tone level to required tone level depending on the optical density and the maximum resistance tolerances.

The following statements relate to the application of the invention with a thermal printing device. Based on the known fact that the large number of thermoelectric printing elements of a given thermal head have a common nominal resistance, but have more or less large resistance tolerances depending on the available manufacturing quality, the consequences for the printing quality are as follows. FIG. 1 shows this a coordinate system in which energy values E are plotted on the abscissa, which are proportional to the heating time of the corresponding pressure element in terms of resistance. The associated values of the optical density OD are plotted on the ordinate. Three resistance curves RU, RN and RO are shown in the coordinate system. The characteristic curve RN represents the nominal resistance of all pressure elements of the thermal head. The characteristic curve labeled RU represents the resistance value of the lower tolerance limit and the characteristic curve RO represents the resistance value of the upper tolerance limit.

The following considerations expediently relate to the characteristic resistance RN of the nominal resistance at five tone levels, represented by 5 levels of the associated optical density OD0 (N) to OD4 (N). It is known that starting from the energy value "0" to an energy value E0 (N) for the characteristic curve RN, the temperature of the printing element is not sufficient to enable ink transfer; this corresponds to the threshold value of the optical density OD0 (N). Increased energy supply up to the energy value E4 (N) leads to a largely linear increase in the optical density up to its level OD4 (N). Any additional energy supply can no longer be converted into increasing optical density; there is saturation behavior.

Between the threshold value of the optical density OD0 (N) and the saturation value of the optical density OD4 (N), three further levels of the optical density OD1 (N) to OD3 (N) are evenly distributed and the energy values E1 (N) required to generate them to E3 (N) for the characteristic curve RN of the nominal resistance.

erzielt wird. Das heißt, für Druckelemente, deren Widerstandswerte der Kennlinie RU zuzuordnen sind, sind nur vier der geforderten fünf Stufen optischer Dichte OD0(U) bis OD3(U) verfügbar, die mit den geforderten Stufen optischer Dichte OD0(N) bis OD4(N) nur am Schwellenwer $\text{t OD0(U) = OD0(N)}$

und am Sättigungswert $\text{OD3(U) = OD4(N)}$

übereinstimmen.It can be seen that for a pressure element whose resistance meets the conditions of the lower tolerance limit and is therefore to be assigned to the characteristic curve RU and to which the energy value E3 (N) is supplied in order to generate the optical density OD3 (N), the saturation value of the optical density $\text{OD3 (N) = OD4 (N)}$

is achieved. This means that only four of the required five levels of optical density OD0 (U) to OD3 (U) are available for printing elements whose resistance values can be assigned to the characteristic curve RU, those with the required levels of optical density OD0 (N) to OD4 (N) only at the threshold $\text{t OD0 (U) = OD0 (N)}$

and the saturation value $\text{OD3 (U) = OD4 (N)}$

to match.

It can further be seen from FIG. 1 that a pressure element, the resistance of which fulfills the conditions of the upper tolerance limit and is therefore to be assigned to the characteristic curve RO, and to which the energy value E4 (N) is supplied in order to generate the optical density OD4 (N) reached the saturation value, but an available optical density OD4 (O), which corresponds to the order of magnitude of the required optical density OD3 (N).

According to the invention, for a number m of physically available tone levels of, for example, m = 15, represented by the discrete energy values E1 to E15, the number n of required tone levels TS1 to TS5 is limited to, for example, n = 5, each of which has a required optical density OD ( TS1) to OD (TS5) is assigned.

Furthermore, provision is made to specify a number k of resistance ranges so that the entire tolerance range of possible resistances is covered uniformly. For example, for k = 50 resistance ranges, for thermal heads whose pressure elements have tolerances of up to ± 20% based on the nominal resistance, each individual resistance range has a resolution of less than 1% based on the nominal value and thus a maximum of 1% deviation within each stage of the optical density. It is known that deviations in the optical density of the order of 1% can no longer be perceived by the human eye and appear to be uniform.

For the sake of clarity, only the characteristic curves R1 and R50 of two resistance ranges are given in FIG. 2, which represent the lower tolerance limit, characteristic curve R1, and the upper tolerance limit, characteristic curve R50. The characteristic curves R2 to R49 of the other k-2 resistance ranges would be arranged between the specified characteristic curves R1 and R50. The characteristic curve R1 in FIG. 2 thus corresponds to the characteristic curve RU in FIG. 1 and the characteristic curve R50 in FIG. 2 to the characteristic curve RO in FIG. 1.

For each of the energy values E1 to E15, the associated available optical density VOD is determined for each resistance range R1 to R50 and recorded in a table. According to the representation in the first quadrant of FIG. 2, for example the available optical density VOD1 (R1) is exactly that which arises when an energy value E1 is applied to a pressure element belonging to the resistance range of characteristic curve R1 and the available optical density VOD2 (R1) is exactly that which corresponds to the energy value E2 with the same resistance range.

As can be seen, the available optical density VOD2 (R1) is just larger than the required optical density OD (TS1) and the available optical density VOD1 (R1) is just smaller than the required optical density OD (TS1). Accordingly, pressure elements which belong to the resistance range of the characteristic curve R1 are alternately acted upon with the energy values E1 and E2 in order to generate pressure points of the optical density OD (TS1). This relationship is indicated in the fourth quadrant of the illustration in FIG. 2 in relation to the required tone level TS1 by two points on the intersection of the lines for the energy values E1 and E2 with the line for the required tone level TS1.

As can be seen from FIG. 2, two successive energy values En and En + 1 (0 <n <15) can always be specified, the alternating application of which to the given pressure element is suitable for realizing one of the required tone levels TS1 to TS5. These energy values are exemplified for pressure elements whose resistances belong to the characteristic curve R1 by points on the corresponding intersection points and for pressure elements whose resistances belong to the characteristic curve R50 by crosses. The assignment of the required tone levels TS1 to TS5 to the energy values E1 to E15 are also determined for each resistance range and recorded in a table.

As can be seen in FIG. 2, the required optical density OD (TS1), for example, is not equal to the mean value from the closest available optical densities VOD1 (R1) and VOD2 (R1). However, in order to achieve exactly the required optical density OD (TS1) on average, an average value deviation MWA is defined for each printing element. The initial value of this mean value deviation MWA (0) is zero. While all other process-related variables are defined once and then remain constant, the current mean value deviation MWA (x) for printing step x is constantly updated as the only variable. The mean value deviation MWA (x + 1) for the following printing step x + 1 results from the sum of the mean value deviation MWA (x) of the current printing step x and the value of the currently required optical density VOD (x) minus the value of the available optical density whose associated tone level is currently being printed.

Für eine geforderte optische Dichte GOD, der der Wert 13,3 zugeordnet sein soll, ergeben sich die unmittelbar benachbarten verfügbaren optischen Dichten OD zu den Werten 13 und 14 entsprechend den Energiewerten E13 und E14. Die Aufeinanderfolge der verfügbaren optischen Dichtewerte für die ersten zehn Druckschritte x ergibt sich zu 14/13/13/13/14/13/13/14/13/13. Nach dem zehnten Druckschritt x ist die Mittelwertabweichung MWA(10) = 0; damit beginnt ein neuer Zyklus gleichen Ablaufs. Der Mittelwert der erzielten optischen Dichte MWO ergibt sich zu:

und ist damit gleich der vereinbarungsgemäß geforderten optischen Dichte VOD.In a first embodiment, if the current mean value deviation is negative, MWA (x) is printed with the available tone level whose available optical density VOD just exceeds the required optical density GOD. The sequence of the first twelve printing steps x is, for example, as follows:

For a required optical density GOD to which the value 13.3 is to be assigned, the immediately adjacent available optical densities OD for the values 13 and 14 result in accordance with the energy values E13 and E14. The sequence of the available optical density values for the first ten printing steps x results in 14/13/13/13/14/13/13/14/13/13. After the tenth printing step x, the mean value deviation MWA (10) = 0; this starts a new cycle of the same sequence. The mean value of the optical density MWO obtained is:

and is therefore equal to the optical density VOD required by agreement.

Die Aufeinanderfolge der verfügbaren optischen Dichtewerte VOD für die ersten zehn Druckschritte x ergibt sich zu 13/13/13/14/13/13/14/13/13/14. Nach dem zehnten Druckschritt x ist erwartungsgemäß auch in dieser Ausführungsform die Mittelwertabweichung MWA(10) gleich Null und es beginnt ein neuer Zyklus. Der Mittelwert der erzielten optischen Dichte MWO ergibt sich zu:

und ist damit gleich der vereinbarungsgemäß geforderten optischen Dichte VOD.In a further embodiment, printing is always carried out with the available tone level, the associated available optical density VOD of which has a value which is closest to the currently required optical density GOD (x) and is at most equal to the sum of the mean value deviation MWA (x-1) previous printing step and the currently required optical density GOD (x). The sequence of the first twelve printing steps is as follows with the same output variables as in the first exemplary embodiment:

The sequence of the available optical density values VOD for the first ten printing steps x results in 13/13/13/14/13/13/14/13/13/14. After the tenth printing step x, the mean value deviation MWA (10) is expected to be zero in this embodiment as well and a new cycle begins. The mean value of the optical density MWO obtained is:

and is therefore equal to the optical density VOD required by agreement.

In both embodiments it can be seen that the sequence of available optical density values 13 and 14 which differ from one another is evenly distributed over a cycle according to the invention. With a print point resolution of 300 dpi, for example, this results in a visual image that gives the impression of a completely uniform optical density, and thus meets the required high quality standards.

In practice, this results in the following printhead-specific workflow. First, all pressure elements are pre-aged evenly by applying a predetermined number of pressure pulses. Then all print elements of a print head are measured outside the device. Each pressure element is assigned to one of the k predetermined resistance ranges and this assignment is tabulated logged. With the device-internal assembly of the printhead, both the printhead-specific and procedural protocols are implemented.

Claims (7)

Method for compensating resistance tolerances when printing a multi-tone image with a printing device which has printing elements arranged in a row and an optical density required by a predetermined number m of tone levels,
characterized,
that - with a number n> m of physically available tone levels (VTS1 ... VTSn), each dot is printed with the tone level whose resistance-specific associated available optical density (VOD) is closest to the required optical density (GOD) and - The deviation of the available optical density (VOD) from the required optical density (GOD) is compensated for, including an immediately adjacent physically available tone level, so that the mean of the actual optical density (MWO) of successive pressure points corresponds to the required optical density (GOD) . Method according to claim 1,
characterized,
that unique a number k of successive resistance ranges is specified in such a way that, based on their resolution, the resolution of the human eye is safely below the nominal value of the resistance, - the resistance value of each pressure element is measured, each pressure element is assigned to one of the k resistance ranges, - a value is assigned to each required (GOD) and each available optical density (VOD), - For each printing element, the difference between the values of the required optical density (GOD) and the currently available optical density (VOD) is defined as the mean value deviation (MWA) and
that cyclical - For each dot (x) to be printed, depending on the current value of the mean deviation (MWA (x)), one of the available tone levels is selected, the optical density of which is adjacent to the required optical density (GOD), and - The mean value deviation (MWA (x + 1)) is determined for the following pressure point (x + 1). Process according to claims 1 and 2,
characterized, - That a sum of the previous mean value deviation (MWA (x-1)) and the value of the currently required optical density (GOD (x)) is formed and - this sum is compared with the value of the currently required optical density (GOD (x)). Process according to claims 1 to 3,
characterized, - That printing is carried out with the available tone level, the available optical density (VOD) of which, given the smaller sum, is just below the required optical density (GOD (x)) and - otherwise the optical density (GOD (x)) just exceeded. Process according to claims 1 to 3,
characterized,
printing with the available tone level, the available optical density (VOD) of which is at most equal to the sum. Method according to claims 1 to 3 and one of claims 4 and 5,
characterized,
that the mean value deviation (MWA (x + 1)) for the following pressure point (x + 1) is determined from the sum minus the value of the available optical density (VOD), with the associated tone level of which the current pressure point (x) is printed . Method according to one of claims 1 and 2,
characterized,
that all print elements of each print head are pre-aged evenly.

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