EP0536526A2 - Supply control for a heating element in a thermal printer - Google Patents

Abstract

A method for controlling the power supply of a thermoprinting heating element with a sequence of current pulses (I) separated according to a print matrix is described. The current pulses (I) trigger a printing process when a predetermined energy content is exceeded and, if appropriate, bring about preheating when this energy content is undershot. At each matrix time point (t1), in each case a printing requirement is determined for a predetermined number of successive matrix time points (t2, t3), the energy content of the current pulses (I1, I2) being progressively increased for matrix time points (t2, t1) without printing requirement lying ahead of a matrix time point (t3) with printing requirement. When the method is used in a thermoprinter, a high printing speed is achieved. <IMAGE>

Description

The invention relates to a method for controlling the supply of a thermal pressure heating element with a sequence of current pulses divided according to a pressure pattern, which trigger a printing process when a predetermined energy content is exceeded and, if this energy content is undershot, possibly cause preheating.

Such a method is used, for example, in a thermal transfer printer, the print head of which has a plurality of printing elements arranged side by side in a row. A heat-sensitive ink ribbon is arranged between the print head and a recording medium to be printed, which transfers a color dot to the recording medium when heated by a heating element above a printing temperature. Between the print head and the record carrier there is a relative movement transverse to the line of the print elements generated. Certain heating elements are then energized at predetermined intervals and a printing process is triggered in each case. The record carrier is thus printed in a grid pattern with characters or an image pattern.

In a method known from DE 38 33 746 A1, the respective heating element is preheated if it does not trigger a printing process. For this purpose, current pulses are supplied to the heating element, the respective energy content of which heats the heating element to a temperature below the printing temperature. The level and duration of the pulse can be controlled depending on the prevailing ambient temperature and the design of the print head. This ensures that the preheat temperature is evenly distributed over the entire printhead area.

In the known method, the energy supplied to each heating element for preheating is set irrespective of whether the heating element in question triggers printing processes frequently or rarely. Since a higher temperature is set at the heating element than at a low pressure frequency, the mean preheating temperature distributed over the entire printhead must be significantly below the printing temperature, so that a perfect printing result is achieved even when the heating element is under high pressure. The consequence of this is that the current pulse supplied to the heating element must have a high energy content in order to trigger a printing process so that the heating element is heated to its printing temperature. The generation of such current pulses is technically complex since the current pulse generator required for this must have a high peak load capacity.

In addition, if there is a large temperature difference between the preheating temperature and the printing temperature, the time for the heating process and the cooling time are also long. These times have a considerable influence on the printing speed that can be achieved with the printing process. Since, in the known method, a sufficiently large temperature difference is required between the preheating temperature, which is uniformly distributed over the entire printhead area, and the printing temperature, the achievable printing speed is limited to a low value.

It is therefore an object of the invention to provide a method for controlling the supply of a thermal pressure heating element with which a high printing speed is achieved.

This object is achieved for a method of the type mentioned above in that a printing requirement is determined for each raster point in time for a predetermined number of subsequent raster points in time, and in that for raster points in time before a raster point in time with printing requirement the energy content of the current pulses is continuously increased without a printing requirement.

The invention is based on the consideration that the printing speed becomes maximum and the energy required to initiate a printing process becomes minimal if the heating element is preheated as close as possible to its printing temperature. Because in this operating state, only a little additional energy is required to trigger the printing process. In addition, the heating up and cooling down times of the heating element are then short. Since the heating element is subjected to different loads over time, depending on the pattern to be printed, the preheating energy supplied to it is increased according to the invention customized his printing program. For this purpose, a preview is determined at each raster point in time whether printing is to take place at subsequent raster points in time. For example, the following two raster times can be included in this preview. This means that the effort for the procedural steps of the preview remains low. If there is no printing requirement for the future grid times, then preheating can be kept to a minimum or even completely eliminated. In this way, the preheating energy for preparing a printing process can only be used when there is actually a print job. This means that energy is saved.

If it is found in the preview that a printing process is to be triggered by the heating element in the near future, the heating element is subjected to current pulses at grid times in which it does not print in order to prepare it. The energy content of the heating element is then continuously increased until the raster time, which is before the raster time with printing requirements, so that the temperature of the heating element at the raster time at which the printing process is triggered is just below the printing temperature. To trigger the printing process, the current pulse then only has to have a low energy content in order to raise the temperature of the heating element from the preheating temperature to the printing temperature. The time required for this is then minimal, as a result of which the printing speed becomes maximum within the operating limits of the thermal print head. Since the current pulse has low energy content, its electrical output is also small, so that the electronic outlay for generating the current pulse remains manageable and an inexpensive hardware solution can be used for the current pulse generator.

The energy content of the current pulses can be increased continuously during the preheating phase, for example by calculating the total energy amount to achieve the desired preheating temperature and adapting the current strength and / or the duration of the current pulses to it.

A preferred embodiment of the invention is characterized in that the energy content is increased in stages. This measure makes it possible to adapt the method in a simple manner to known digital methods for controlling the thermal print head, in which the heating elements are already supplied in discrete, adjustable current intensities or pulse widths.

In a development of the aforementioned embodiment, the current pulses have a constant current intensity, the respective duration of which is continuously extended. The duration of each current pulse can be composed of equally large time intervals.

These measures make it possible to use a simple constant current source to supply the heating element. The duration or pulse width of the current pulse is varied in order to change its energy content. By using equally large time intervals, the circuit complexity can be further reduced when realizing the pulse generation, since the current pulses can be derived from a link with already existing clock pulses of a control center.

Another embodiment is characterized in that in the case of a thermal print head with a plurality of heating elements arranged next to one another at each raster time pressure requirements are determined for a predetermined number of subsequent grid times, and that the increase in the energy content of the current pulses of the respective heating element is reduced when a pressure requirement of a heating element adjacent to it is determined.

In this embodiment, the technical effect is exploited that part of the heat of one heating element is transferred to the adjacent heating element. If an adjacent heating element is also to trigger a printing process within the period under consideration and is therefore preheated to a higher temperature, the energy part which is transferred from this heating element to the respective heating element need not be supplied during the preheating phase. The energy content of the current pulses can then be reduced by this part. This results in an even more economical use of energy.

According to a further preferred exemplary embodiment, the timing of the current pulses in the raster interval differs from the timing of the current pulses for at least one further heating element. This measure can be used advantageously if a thermal print head with a large number of heating elements, for example 256 or 512, is used for printing. For example, when printing postage stamps on envelopes in a franking machine, thermal print heads of this type can be used, since a very wide line of text can be printed with a single feed movement. The measures of the aforementioned embodiment ensure that the heating elements of the thermal print head can be divided into at least two areas. The heating elements of the first area will be then supplied with current pulses slightly delayed to those of the second region. As a result of this time offset, the electrical power to be supplied to the heating elements can be distributed over time and the peak load on the current source supplying the current pulses can be reduced. The printing speed does not have to be reduced.

Fig. 1

eine Frankiermaschine mit einem Thermotransferdruckwerk,

Fig. 2

eine schematische Darstellung eines Druckkopfes mit 5 Heizelementen,

Fig. 3

schematisch den Verlauf der Temperatur eines Heizelements während der Vorheiz- und der Druckphase über der Zeit,

Fig. 4

Stromimpulse für verschiedene Betriebszustände eines Heizelements,

Fig. 5

ein Ablaufschema zum Erzeugen eines Stromimpulses für ein Heizelement zu einem vorgegebenen Rasterzeitpunkt,

Fig. 6

ein Blockschaltbild einer Steuerung zum zeitversetzten Speisen zweier Heizelemente nach dem Multiplexverfahren, und

Fig. 7

die den Heizelementen nach Fig. 6 zugeführten Stromimpulse über der Zeit.

Embodiments of the invention are explained below with reference to the drawings. It shows

Fig. 1

a franking machine with a thermal transfer printing unit,

Fig. 2

a schematic representation of a printhead with 5 heating elements,

Fig. 3

schematically the course of the temperature of a heating element during the preheating and the printing phase over time,

Fig. 4

Current pulses for different operating states of a heating element,

Fig. 5

a flow diagram for generating a current pulse for a heating element at a predetermined raster time,

Fig. 6

a block diagram of a controller for staggered feeding two heating elements according to the multiplex method, and

Fig. 7

the current pulses supplied to the heating elements according to FIG. 6 over time.

In Fig. 1, essential components of a franking machine are shown in a schematic representation, for which the invention is used. A display unit 1 and a keyboard field 2 are arranged on the outside of a housing 3. An externally accessible housing part 4 accommodates an ink ribbon cassette (not shown) for thermal transfer printing. The envelopes to be printed are moved between the underside of a print head carrier 5 and a platen 6, with color particles being transferred from the ink ribbon of the ink ribbon cassette to the envelopes during printing.

A control board 13 is shown on the right in FIG. 1, which contains a microprocessor 8, a working memory 7, a program memory 9, a serial interface 14, a printhead connection 16, a service interface 17, a customer-specific read-only memory 12 and a power supply connection 18.

In the lower part of FIG. 1, a printing unit 10 is shown schematically, which is held by the print head carrier 5. The printing unit 10 contains a thermal transfer print head 11 and a transport motor 21 for driving a transport roller 22 which conveys the envelopes past the print head 11.

2, the print head 11 is shown schematically. On the side facing the ink ribbon, it has heating elements R1 to R5 which are arranged next to one another in a row and are formed by electrical resistors. For printing, a relative movement of approximately constant speed is generated between a recording medium 30, for example an envelope, and the print head 11. At grid times t1 to t5 with At the same time intervals tp, the heating elements R1 to R5 along the webs 32 to 40 can trigger printing processes in which the color of the ink ribbon is transferred to the recording medium 30, for example at points 42 to 50.

Fig. 2 shows only schematically the printing principle used here. In reality, the print head 11 consists of considerably more heating elements, for example 256 or 512 heating elements, which are arranged next to one another at short intervals. With the aid of such a printhead, an approximately 60 mm wide section can be printed with a single relative movement between the recording medium 30 and the printhead 11, as is e.g. is required for the automatic franking of envelopes.

The method according to the invention for controlling the supply of a heating element is described in more detail below with reference to FIG. 3 using the heating element R4, which is intended to trigger a printing process at raster time t3 in accordance with FIG. 2. In this figure, the temperature T is plotted over time t in the upper part of the figure. At the raster point in time t1, the heating element R4 has a temperature which essentially corresponds to the ambient temperature Tu. According to the embodiment of the invention described here, three raster times t1, t2 and t3 are examined to determine whether a printing process should be triggered. It is determined here that such a printing requirement is present at raster time t3. At grid times t1 and t2, heating element R4 is now preheated in preparation for the printing process by continuously increasing the energy content of current pulses I1 and I2 supplied to electrical resistance of heating element R4 at time intervals tp.

In the lower part of FIG. 3, the current pulses t1 to t3 belonging to the times t1 to t3 are shown over the time t. Their respective energy content is set by the pulse duration. The current pulse I1 supplied at raster time t1 causes the heating element R4 to raise the temperature to well below the limit temperature Tg above which a color transfer from the heat-sensitive ink ribbon to the recording medium 30 takes place, i.e. a printing process is triggered. Due to heat dissipation to the environment, the temperature drops again until the raster time t2, but remains significantly above the ambient temperature Tu.

At raster time t2, the heating element R4 is supplied with a current pulse I2 with a higher energy content than that of the current pulse I1, so that the temperature rises to close to the limit temperature Tg. If the printing process is to be triggered at raster time t3, the energy content of the corresponding current pulse I3 only has to be increased slightly so that the limit temperature Tg is exceeded. Thus, in the method according to the invention, the heating element R4 is preheated to close to the limit temperature Tg, so that the actual current pulse I3 for triggering the printing process can be minimized in its energy content and thus in its duration. This enables a high repetition frequency of the printing process and thus a high printing speed.

4 shows various operating states a) to f) for the grid times t1 to t3 and the associated ones Current pulses are shown, which are supplied to the heating element R4 at the raster time t1. Circles in the left part of the image indicate whether a printing process should be triggered at the raster times t1 to t3. An empty circle indicates that no printing process should be triggered, a hatched circle indicates that there is a printing requirement. In operating state a), no printing process is to be carried out for the raster times t1 to t3. No current pulse is supplied to the heating element R4 at the raster point in time t1. There is no preheating.

In operating state b), a printing process should take place at raster time t3. In order to preheat the heating element R4 sufficiently, a current pulse of constant intensity is supplied to it at the raster time t1. The energy content of the current pulse is set by its pulse width or duration. For this purpose, the current pulse is composed of partial pulses, each of which is a time interval T long. In operating state b), a current pulse of duration 2T composed of two partial pulses is accordingly generated and supplied to the heating element R4.

In operating state c) there is a printing requirement at raster time t2. To preheat to a temperature value just below the limit temperature Tg, an increased energy content of the current pulse is required, which is realized by extending the pulse duration to four time intervals T.

In the operating state d), a printing process should already be triggered at the raster time t1. In order to exceed the required limit temperature Tg, a current pulse of duration 5T is supplied to the heating element R4. If again in the subsequent raster time t2 If printing is to be carried out (not shown in the figure), the heating element R4 is again subjected to a current pulse of duration 5T. The operating state of the heating element, in which it continuously triggers printing processes, for example in order to produce a solid line on the recording medium, determines the maximum achievable printing speed of the print head 11. Since the duration of the current pulses for triggering printing processes is minimized by the method according to the invention, a high printing speed is achieved.

To determine the energy content of the current pulses, only 3 raster times t1 to t3 were considered in this example and the current pulse for triggering a printing process was set to 5 time intervals T. In practice, it has been shown that a considerable advance in printing speed is already achieved without the computing effort for determining the printing requirements and the hardware expenditure for individually preheating the heating element R4 being great. It is easy to see that the inclusion of further raster times in the preview and a finer gradation of the pulse duration through a higher number of partial pulses enables an even better use of the potential for increasing the printing speed within the operating limits of a print head.

In the operating state e), the operating state of the adjacent heating elements R3 and R5, along the paths, is also taken into account when determining the energy content of the current pulse to be supplied to the heating element R4 36 and 40 (see FIG. 2) are arranged. At raster time t3, the heating elements R3, R4 and R5 should each trigger a printing process. Since all three heating elements R3, R4, R5 are preheated and thus assume a higher preheating temperature than the ambient temperature Tu, less energy flows from the heating element R4 to the environment than without preheating the neighboring heating elements R3 and R5. The energy content of the current pulse to be supplied to the heating element R4 can be reduced by this amount. Therefore, the heating element R4 is supplied with a current pulse of reduced duration 1T instead of a current pulse of duration 2T (cf. operating state b)) at raster time t1. The same happens if only one of the heating elements R3 or R4 is to trigger a printing process at raster time t3.

In operating state f) there is a print requirement at raster time t2. For this operating state, the duration of the current pulse supplied to the heating element R4 at the raster time t1 is set to 3T. The same procedure is followed if only one of the heating elements R3 or R4 is to trigger a printing process at raster time t2.

A process flow for controlling the supply of the heating element R4 is shown schematically in a flowchart in FIG. 5. Such a procedure can e.g. by executing a program using the microprocessor 8 (see FIG. 1). In the method steps described in more detail below, the operating states shown in FIG. 4 are recognized and the associated current pulses are output

In method step 60, it is determined from the print data transmitted to print head 11 whether it is current Raster time t1 for the heating element R4 there is a pressure requirement. If this is the case, the method branches to step 66 in step 62. A partial pulse of duration 1T, ie a time interval T, is generated in this. Otherwise, proceed to method step 64. This analyzes whether there is a pressure requirement for the heating element R4 at the raster time t2. If the answer is affirmative, the method branches to step 70 in step 68 and determines whether one of the adjacent heating elements R3 or R5 or both should trigger a printing process. If this is the case, then a further partial pulse of duration 1T is generated in method step 72. Otherwise, method step 72 is omitted. In the subsequent method step 73, a partial pulse is again generated.

It is then determined in method step 74 whether there is a pressure requirement for the heating element R4 at the raster point in time t3. If the result is positive, the method branches to step 78 in step 76. In this it is examined whether a printing process should be triggered simultaneously with the neighboring heating elements R3 or R5 or with both at raster time t3. If this is the case, a further partial pulse of duration 1T is generated in method step 80. Otherwise, step 80 is omitted and a partial pulse of duration 1T is generated in step 82.

If no pressure requirement has been determined in method step 76, a branch is made to method step 84 and the current pulse composed of the partial pulses generated in steps 66, 72, 73, 80 and 82 is output. This can last from 5T to 0T (empty pulse).

In Fig. 6, the control of the supply of heating elements according to the timing method is shown schematically in a block diagram. The heating elements of the print head 14 are electrically divided into two areas T1 and T2. They are fed with current pulses at different times via a multiplexer 92. The switch position of the multiplexer 92 is controlled by the microprocessor 8. A pulse generator 90 generates 8 current pulses depending on data from the microprocessor, which pulses are fed to the multiplexer 92. By using the timing method, it becomes possible to feed a large number of heating elements of a printhead 11, for example 512 heating elements, with an electrically simple current pulse generator 90, since its peak current load is reduced by the time shift.

7 shows the current profiles over time t for one heating element in each of the areas T1 and T2. As can be seen from the figure, the corresponding raster instants t1 and t2 are offset by 5 time intervals T, i.e. the duration of a current pulse to trigger a printing process. This results in a pulse-pause ratio or duty cycle of 50% for each area T1, T2. Since the pulse duration of a current pulse for triggering a printing process is minimal in the method according to the invention, the advantage of the high printing speed can be fully exploited when using the timing method.

Claims (11)

Method for controlling the supply of a thermal printing heating element with a sequence of current pulses divided according to a printing pattern, which trigger a printing process when a predetermined energy content is exceeded and, if this value is undershot, possibly cause a preheating, characterized in that a predetermined one at each timing point (t1) Number of subsequent raster instants (t2, t3) a pressure requirement is determined in each case, and that the energy content of the current pulses (I1, I2) is continuously increased for raster instants (t2, t1) prior to a raster occurrence point (t3) with a printing requirement. A method according to claim 1, characterized in that the energy content is increased in stages. Method according to Claim 2, characterized in that the current pulses (I1, I2, I3) have a constant current and their respective duration is continuously extended. Method according to claim 3, characterized in that the duration of each current pulse is composed of equally large time intervals (T). Method according to claim 4, characterized in that the duration of the current pulse (I3) for triggering a printing process is 5 time intervals (T). Method according to one of the preceding claims, characterized in that at each raster point in time (t1) for two subsequent raster points in time (t2, t3) the printing requirement is determined in each case, and that in the absence of a printing requirement no current pulse is output. Method according to one of the preceding claims, characterized in that the sequence of current pulses (I1, I2, I3) has a constant period (tp). A method according to claim 7, characterized in that the time interval (T) is an integral multiple, preferably 5 times, smaller than the period (tp). Method according to one of the preceding claims, characterized in that, in the case of a thermal print head (11) with a plurality of heating elements (R3, R4, R5) arranged next to one another, printing requirements are determined at each raster time (t1) for a predetermined number of subsequent raster times (t2, t3) , and that the increase in the energy content of the current pulses (I1, I2) of the respective heating element (R4) is reduced if a pressure requirement of a heating element adjacent to it (R3, R5) is determined. Method according to one of the preceding claims, characterized in that the time position of the current pulses in the raster interval (tp) is different from the time position of the current pulses for at least one further heating element. Method according to Claim 10, characterized in that the time difference is half a grid interval (tp).

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