EP0140389B1 - Method and apparatus for the reduction of the print offset in a bidirectional matrix printer - Google Patents

Description

The invention relates to a method for reducing the printing offset in bidirectionally printing matrix printing devices according to the preamble of claim 1, and an arrangement for performing the method.

Printheads from matrix printers are known to have printing elements, for example needles, which are arranged in one or more rows. The control of the individual printing elements with information contained in a character generator takes place in columns, i. H. after the printhead has moved a column distance, the printhead is fed a column and print cycle. The pressure cycle thus determines the time of actuation of the pressure elements. In this way, each character can be built up in a grid pattern on the record carrier.

As is generally known, the printing processes always take place during the movement of the print head at a constant speed. The formation of the print cycle can take place by scanning a so-called printer carriage carrying the print head or the drive device driving it. In particular, this is done by deriving the pressure clocks from the scanning clocks formed in the scanning device, which is generally implemented by an optically scanned clock disk or clock ruler. For this purpose, a so-called PLL module can be used, which converts the sampling clock applied to the input into a higher-frequency output clock while largely maintaining the phase relationship. The output clock can then be used in a pressure clock to form the pressure clock. This clock generation known for matrix printers makes it possible, by counting the print cycles, starting from the start of printing, i. H. starting at the beginning of the line, to determine the print cycles of a character or the print cycles of an entire print line. This is important insofar as the movement of the printer carriage must be braked and then accelerated again in the other direction of movement. As mentioned above, the print cycles are only available during the constant movement of the printer carriage. The sampling clocks are therefore used during the deceleration and acceleration phases, i. H. here these are counted

The problem arises that errors can occur at the end of a printing process, namely when switching the counting of printing cycles to the counting of scanning cycles, which lead to a printing offset. Such errors are due to the fact that effects of the mechanical system parts, such. B. the belt stretching with toothed belt drive, the needle flight time with needle printheads etc. or unavoidable tolerances of components lead to the fact that the last pressure cycle is in the immediate vicinity of the flank of a scanning cycle, so practically random, d. H. can no longer be influenced before or after an edge of the sampling clock. This then leads to the mentioned pressure offset in the impression, i. H. to a poor printed image, which has a particularly unfavorable effect in the case of bidirectional printing, that is to say when printing in the forward and reverse directions.

The object of the invention is to avoid such a printing offset in the case of bidirectionally printing matrix printers.

This object is achieved in accordance with the characterizing features of patent claim 1. Embodiments of the invention and an arrangement for carrying out the invention are specified in the subclaims.

• Fig. 1 die für einen Matrixdrucker nach dem Stand der Technik vorgesehenen Taktfolgen,
• Fig. 2 die Taktfolge mit erfindungsgemäß eingefügten Leerdrucktakten und
• Fig. 3 ein Ausführungsbeispiel in Form eines Blockschaltbildes zur Durchführung des Verfahrens nach der Erfindung.

To explain the invention, reference is made to the drawings. Show there

• 1 shows the clock sequences provided for a matrix printer according to the prior art,
• Fig. 2 shows the clock sequence with blank pressure clocks inserted according to the invention and
• Fig. 3 shows an embodiment in the form of a block diagram for performing the method according to the invention.

The illustration in FIG. 1 is based on a mosaic printing device in which a scanning clock AT, a control clock ST and a printing clock DT are formed when the printer carriage carrying a print head moves along a write line. The sampling clock AT, the control clock ST and the pressure clock DT are shown in lines 1, 2 and 3 in FIG. 1. The sampling clock AT is determined by the scanning of the printer carriage carrying the print head or by the scanning of a drive, e.g. B. formed with the drive motor timing disc. Starting from the state of the printer carriage, the frequency of the sampling clocks AT increases continuously during the acceleration phase until the printer carriage has reached the speed required for the printing process. This point in time and thus also the position of the printer carriage is determined in a known manner by counting the sampling clocks in a counter. It would also be possible to determine this position of the printer carriage using a so-called line start contact. 1, the acceleration phase is ended at time t1. Defined control clocks ST are available from time t1 and are generated, for example, in a PLL circuit known per se. In the example shown, the control clock ST has an 8 times higher frequency than the scanner AT. The pressure cycle DT in turn is formed by dividing the control cycle ST by a factor of 3 in the example. During the printing phase, the column clocks required for character printing in a writing line are output for the printhead, it being assumed for this example that the Dar position of each character 32 column clocks are required. Accordingly, the print cycle DT32 of the last character in a line is also the last print cycle of this line. In the example according to FIG. 1, this occurs at time t2. Depending on this, the rising edge of the next following sampling cycle AT for initiating the braking process for the printer carriage is evaluated. To do this, the required number of sampling clocks AT is counted. During the subsequent acceleration phase in the reverse direction, the processes described above run again until the printer carriage has reached the prescribed speed again and the printing cycles for the printing process are output in the reverse direction.

Due to deviations of the control clocks ST and the pressure clocks DT caused by component tolerances or due to deviations when scanning a clock disc, as well as due to influences caused by the drive elements (e.g. belt stretch), the last pressure clock DT32 can be in the immediate vicinity an edge of the sampling clock AT occur and once shortly before, another time shortly thereafter. This means that an error occurs at the end of printing when switching from the counting of the printing clocks to the counting of the scanning clocks, which error can comprise a clock period of the scanning clock. This error manifests itself in a clearly visible pressure offset, i. H. the typeface looks very irregular. In the case of printing over several lines, this can even lead to a loss of information, if the printing operations are outside the limits of the recording medium. Such a shifted last print cycle DT32 has been entered in dashed lines in FIG. 1. In the case shown, the printer carriage would continue to run at a constant speed during the scanning cycle following the last printing pulse, and the braking processes would be initiated late by an entire scanning cycle period.

If, as proposed by the invention, one or more additional pressure cycles without print information, so-called blank pressure cycles, are added to the last actual pressure cycle, it is achieved that the last of these pressure cycles is a sufficiently large distance from the falling or rising edge of the next, which is provided as the evaluation edge Has sampling clock. For the evaluation of the subsequent, in the example, the rising edge of the next sampling clock that then follows, the same conditions therefore apply to all print lines and a print offset no longer occurs. This becomes clear from the illustration in FIG. 2. There, the sampling clocks AT are again shown in line 1, the control clocks ST in line 2 and the pressure clocks DT in line 3. However, this representation only covers the area of the last print cycle. According to the information given with reference to FIG. 1, the last real pressure cycle DT32 occurs at time t2. Following the last actual pressure cycle DT32, two additional blank pressure cycles DTL1 and DTL2 are now added in the example. In practice, this means that the .start of the braking phase is delayed in a defined manner, which is represented in FIG. 2 by the delay time v. The last of these pressure cycles, namely the pressure cycle DTL2, is at a sufficiently large distance from the rising edge of the next or the previous scanning cycle AT to ensure that the braking process is carried out safely.

After braking the printer carriage, reversing the movement and then accelerating the printer carriage in the reverse direction, the idle cycles are also taken into account. When the speed of the printer carriage intended for the printing process has been reached, the previously added empty print clocks DTL1 and DTL2 are inserted again, so that the print in the backward direction begins at the point at which the last character of the previous line was printed.

The addition of one or more blank pressure cycles makes it possible not only to compensate for deviations in the control and pressure cycle formation; it is also possible to take into account the influences that can change the sampling clocks. In these cases, too, adequate security against a printing offset is ensured with the addition of blank pressure clocks according to the invention.

A possible exemplary embodiment for carrying out the invention is shown in FIG. The drive system for the printer carriage DR basically consists of a motor M, which can be switched in its direction of movement. The drive system is assigned a scanning device AS, which is designed, for example, in the form of a clock disk, the opaque and translucent areas of which are scanned by means of an optoelectronic device. The scanning clocks AT formed in this way represent the information required for determining the movement of the printer carriage. The control clocks ST and the printing clocks DT are generated in a manner known per se by means of counting and clocking units. Specifically, this is a first counter Z1, to which the sampling clocks AT are available. The control clock ST is formed in a control clock generator STG, which is implemented by a so-called PLL circuit, from the sampling clocks AT. The higher-frequency control clock ST arrives in a pressure clock generator DTG, from which the actual pressure clocks DT are emitted. These are also fed to a second counter Z2, which counts the pressure cycles DT and in this way recognizes the last pressure cycle DT32. The counters Z1 and Z2, as well as the pressure pulse generator DTG exchange corresponding control criteria S1 to S6 with a control part S. The individual tax criteria are as follows. With the start of the movement of the printer carriage DR the control signal S1 is given to the counter Z1, which then counts the sampling clocks AT required for the acceleration of the printer carriage. If the printer carriage DR has reached the prescribed speed, ie if the counter Z1 has reached a prescribed counter reading, then the control signal S2 is output to the controller S. Thereupon, the control clock generator STG and the pressure clock generator DTG are now switched on by means of the control signals S3 and S4, and the second counter Z2 is switched on by the control signal S5. The pressure clock generator DTG, to which the control clock ST is available, then forms pressure clock cycles DT corresponding to the structure and the How the matrix printer works. If the counter Z2 has recognized the last print cycle DT32 of a line, it outputs the control signal S6 to the control system S. This reacts to this in such a way that the switch-off of the control clock generator STG, the pressure clock generator DTG and the activation of the counter Z1 is delayed. This means that the pressure clock generator DTG continues to output pressure cycles after the recognition of the last pressure cycle DT32, which however, as empty pressure cycles DTL1, DTL2, do not lead to an impression on the recording medium P. After the output of one or more such idle pressure cycles, the control signal S3 is switched off by the control S and the control signal S1 is switched on again. This means that now no more printing clocks are issued and that the counter Z1 counts the sampling clocks AT again. The braking process and then another acceleration process in the reverse direction are thus carried out. If the printer carriage DR has once again reached the prescribed speed, ie if the counter reading of the counter Z1 corresponds to the prescribed value again, the control signal S2 is output to the controller S. The control clock generator STG and the pressure clock generator DTG are switched on by the control signals S3, S4 in order to emit the corresponding number of idle pressure cycles. If these have been transferred to the print head, the counter Z2 is switched on and the subsequent print cycles are evaluated in the print head for the representation of characters and at the same time counted in the counter Z2. If the last print cycle of the line is determined in this way, the processes described are repeated.

Claims (2)

1. Method for the reduction of the print offset in a bidirectional matrix printer, comprising a printer carriage (DR) which can be moved along a recording medium (P), characterized in that the print clock signals (DT) are derived via a print clock generator (DTG) from a scan clock (AT) formed by the scanning of a scanning device (AS) and in which the scan clock signals (AT) are counted during the acceleration phase and the . deceleration phase of the printing mechanism (DW) and the print clock signals are counted during the actual printing process and are evaluated by a control section (S), in that, in dependence on the last print clock signal (DT32) intended for the printing of a character of a printed line, at least one further print clock cycle (DTL1, DTL2) without printing information (blank print clock signal) is generated following this print clock signal (DT32), in that the number of further print clock signals (DTL1, DTL2) without printing information is determined by the distance between the last print clock signal (DT32) provided for printing and the rising edge of the next scan clock signal (AT), a minimum distance being maintained between the last print clock signal (DTL2) without printing information and the rising edge of the next scan clock signal (AT), which distance ensures a faultless transmission of the counting of the print clock signals (DT) to the scan clock signals (AT) at the end of printing, in that the deceleration of the printer carriage (DR) is initiated after the further or the last of the further print clock signals (DTL1, DTL2), and in that, after the reversal of the direction of movement of the printer carriage (DR), these print clock signals (DTL1, DTL2) are again inserted for the formation of the first print clock signal (DT1).

2. Arrangement for carrying out the method according to Claim 1, characterized in that the control system (S) is connected to counters (Z1, Z2) for the scan clock signals (AT) and for the print clock signals (DT) and to a control clock generator (STG) and to a print clock generator (DTG), in that the control system (S) emits after the start of a movement of the printer carriage (DR) a first control criterion (S1) to the first counter (Z1), by means of which the number of scan clock signals (AT) for the acceleration phase is determined and receives from the counter (Z1) a second control criterion (S2) when the constant printing speed is reached, in that the control system (S) thereupon emits a control signal (S3, S4, S5) which switches on the control clock generator (STG), the print clock generator (DTG) and the second counter (Z2) by means of which the number of print clock signals (DT) for the printing process is determined and receives from the second counter (Z2) a further control criterion (S6) after the last print position of a line has been reached and switches off the control clock generator (STG), the print clock generator (DTG) and the first counter (Z1) with such a delay (v in Fig. 2), that the print clock generator (DTG) emits in dependence on the initiation of a minimum distance from the rising edge of the scan clock signals (AT) at least one further print clock signal (DTL) without printing information.

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