EP0134258B1 - Hammer flight time control for an impact printer - Google Patents

Description

The invention relates to a device for monitoring the flight time of the print hammers of impact printers, in which the printing process is carried out by moving and striking a print hammer against a printing medium carrier passing by and a stop plate, the flight time being controlled by a control signal derived from the triggering time and impact time of the respectively monitored hammer Measuring device is determined.

The invention can be used for printing units which are known and used in large numbers, in particular in the technology of data processing as output devices, so-called fast printers. The print media used include: Steel strips with etched types or point elements or type wheels are considered.

In order to simplify the illustration of fast printers, a tape is used as the type carrier in the following, but the explanations as well as the invention relate to all types of impact printers, where individual print hammers strike the moving type carrier.

The type carrier, i.e. the type band, runs continuously in the direction of the print line. The number of printing points on the recording medium to be printed is determined by the number of printing hammers, provided that these cannot be moved in the line direction. Every print type is offered at every print point of a print line. Each pressure point is assigned a stop device that releases a print hammer at the desired time, i.e. when the type of print to be printed is at the relevant printing point.

The time of flight of the print hammer is the time that lies between the triggering time of the print hammer and the time at which the print hammer hits the recording medium or the stop plate.

As is known, the flight times of the print hammers in such high-speed line printers change over time. Possible causes are wear of the surfaces of the pressure hammer or wear of the actuating element, change in the flight distance to be covered, changes in mechanical friction, fluctuations in the electrical voltage at the driver supplying the electrical trigger signal, temperature fluctuations and some others.

The flight time changes have a direct impact on the print quality.

Because the frequency with which the individual hammers are used in particular has an impact on flight times. which therefore differs from print position to print position, it is necessary that the flight times of all print hammers are adjusted to one another by adjusting the hammer flight paths at certain time intervals or when high demands are placed on the print quality, even at the request of the user of the fast printer. This manual adjustment is costly and does not prevent print quality from deteriorating between two adjustments.

It would be of great advantage if the manual could be replaced by an automatic, processor-controlled adjustment. Different physical flight times could then be accepted and only the triggering times of the individual print hammers could be shifted accordingly. By shifting these triggering times, it is achieved that the times of impact of each printing hammer fit into the chronological grid of the printing sequence. In order to be able to shift the triggering time of the individual print hammers controlled by a micro program, the exact time of impact of each hammer must be determined on the recording medium or the stop plate.

From DE-OS 19 32 560 it is already known to carry out a static error compensation of the flight time differences of the individual print hammers by dynamic error compensation of deviations of different sizes, which change from time to time, automatically. The error compensation is then carried out by comparing a reference signal with a control signal derived from the hammer movement and deriving a correction signal.

A similar arrangement for keeping the flight time of the print hammers constant can be found in DE-OS 18 15 239. The flight time of the print hammers serves as a controlled variable, which is influenced via the switch-on time of the print hammer drives, which is effective as a control variable, in such a way that the disturbing variables occurring are compensated for.

An arrangement for monitoring the movement of printing hammers can also be found in DE-OS 24 02 895, where a separate displacement sensor is provided for each printing hammer, which detects the hammer movement without contact. The temporal correspondence of the encoder signal with a desired signal characterizing the correct time range is used in a monitoring circuit to compensate for the differences in flight time.

Finally, DE-OS 23 53 825 shows an arrangement for aligning the flight times of printing hammers, in which the time of flight correction is compensated for by delay elements in that corresponding correction signals are derived via a stop detector.

• 1. Man druckt von Zeit zu Zeit eine Testzeile. Dabei erweist es sich als sehr nachteilig, daß dem Benutzer des Druckers zugemutet werden muß, daß zwischen seinen Ausdrucken gesondert Testzeilen hinzunehmen sind. Dabei bringt auch das Ausrücken des verwendeten Farbbandes keine Abhilfe, da oft mit Durchschlagpapier gearbeitet wird.
• 2. Man verzichtet auf derartige Testausdrucke und registriert während des normalen Druckens die Flugzeiten der einzelnen Druckhämmer. Dabei ist es von Nachteil, daß Flugzeiten von Druckhämmern in Druckpositionen, die nicht benutzt wurden, auch nicht justiert werden können. Außerdem überlappen sich die Flugzeiten der einzelnen Flughämmer, so daß ihre Anschlagzeitpunkte nicht ohne weiteres den Druckpositionen zuzuordnen sind, wenn nicht für den Druckhammer ein gesonderter Geber oder Fühler für den Anschlagzeitpunkt vorgesehen wird. Gerade die letztere Maßnahme bedingt jedoch einen nicht unbeträchtlichen Aufwand und hat oft Störungen der sonstigen Funktionen des Druckers zur Folge.

These known devices for monitoring the flight times of the print hammers of impact printers offer two options in particular.

• 1. You print a test line from time to time. It proves to be very disadvantageous that the user of the printer must be expected to add separate test lines between his printouts. The removal of the ribbon used does not help, since carbon paper is often used.
• 2. You do without such test printouts and record the flight times of the individual print hammers during normal printing. It is disadvantageous that the flight times of printing hammers in printing positions that have not been used cannot be adjusted either. In addition, the flight times of the individual hammers overlap, so that their times of attack cannot be easily assigned to the printing positions unless a separate sensor or sensor is provided for the time of attack. However, the latter measure requires considerable effort and often results in malfunctions in the other functions of the printer.

The invention seeks to remedy this. The invention, as characterized in the claims, achieves the object of specifying a device for monitoring the flight time of print hammers of impact printers, which enables automatic self-adjustment of the printer without the user expecting test prints, without impairing the other functions of the printer and without taking expensive measures in the area of each printing position.

The advantages achieved by the invention are essentially to be seen in the fact that it is possible to do without any mechanical adjustment of the print hammers, so that the maintenance time of printers is drastically reduced, that the test cycles leave no visible traces on record carriers, that the invention also applies Chain printer, type wheel printer, etc. can be used without significantly changing the device and that, in addition to a stop detector and a slightly changed print type carrier, only a small amount of memory space in the microprocessor and corresponding program parts are required.

• Fig. 1 schematisch einen Teil eines als Drucktypenträger dienenden Stahlbandes, das neben normalen Drucktypen eine erfindungsgemäße Testtype aufweist,
• Fign. 2A und 2B spezielle Verbindungen der Testtype mit dem Drucktypenträger und
• Fig. 3 ein Blockschaltbild für eine Flugzeitüberwachung und automatische Flugzeitjustage der Druckhämmer eines Zeilendruckers.

The invention is explained in more detail below with the aid of drawings which illustrate only one embodiment. Show it:

• 1 schematically shows a part of a steel strip serving as a printing type support which, in addition to normal printing types, has a test type according to the invention,
• Fig. 2A and 2B special connections of the test type with the print type carrier and
• Fig. 3 is a block diagram for time of flight monitoring and automatic time of flight adjustment of the print hammers of a line printer.

The steel strip 20 serving as a printing type support, as is shown schematically in FIGS. 1, 2a and 2b, has raised printing types 21 designed in a mirror-image form. The printing types 21 are produced, for example, by etching the steel strip. In addition to these printing types, there are also raised position markings 22 produced by etching on the printing type support 20, which mark the individual printing types or the position of the ribbon during its passage through the printing unit. In addition to the normal print type set, there is at least one test type 23 on the print type support 20 according to the invention. The decisive feature of this test type 23 is that although it is raised in the same way as the print types 21 themselves, it has an impact surface that is greater than that of the print types 21 is significantly enlarged. The impact area of the test type is so large that a hammer impact does not result in an imprint on the recording medium to be printed, for example a paper web, due to the small surface pressure which arises. An impression is only created if the raised area of a print type differentiates a certain value, eg 1 mm ² . With an area of the test type corresponding to the usual area of the print hammers used for striking, for example 2.5 x 5 mm ² , the area load would be twelve times smaller than that required for printing, so that no impression is made on the impact. The test type can have a multiple of the normal print type width / print type height.

In the exemplary embodiment according to FIG. 1, the test type 23 is connected with the entire rear side to the print type support 20, since like the print types themselves it is produced by etching the steel strip. This type of attachment of the test type 23 can lead to problems, since the flexibility of the print type support can no longer be guaranteed due to the large area and the rigid connection to the print type support. The flexibility must be guaranteed, since the steel strip used as the type carrier is designed as a closed loop, and must be guided around the printing station in constant circulation and must therefore be constantly deflected.

To maintain flexibility, test type 23 can be divided into subfields. The exemplary embodiment of the test type 23 shown in FIG. 2A is characterized in that the test type is only firmly connected to a web connection 24, which in turn is only resiliently connected to the print type support 20 via the end 25. Flexibility is also guaranteed here.

In the exemplary embodiment according to FIG. 2b, the test type 23 is only connected to the printing type support 20 on one side via a weld seam 26, so that the flexibility is also retained here.

The mode of operation of the test type 23 described is as follows described with reference to FIG. 3. First of all, the actual printing station of an impact printer is shown purely schematically, that is to say in the example of a line printer. First of all, there is a printing hammer bank with a printing hammer 6 for each possible printing position, for example 132 printing hammers. The print hammer bank is arranged opposite a stop plate or stop rail 1. Between the print hammers 6 and the stop rail 1, starting from the stop rail 1 in parallel guidance, the print type carrier 20, an ink ribbon 3 and the record carrier to be printed, for example a paper web 4, run through the printing station. Whenever a certain print type reaches a certain print position, the print hammer 6 assigned to this print position is triggered, strikes against the paper web 4 and thus via the ink ribbon 3 against the print type support 20 and the stop rail 1. The desired print on the paper web 4 is produced. The triggering of the individual print hammers 6 takes place via respectively assigned triggering circuits 7, which are not described in greater detail and which are controlled via printer electronics 12. The printer electronics 12 itself is in turn via one of those shown in FIGS. 1, 2A and 2B indicated position markings 22 controlled on the sensor 13 sensing the print type carrier 20.

In connection with the known printing station described above, the device according to the invention for monitoring the flight time of the print hammers 6 and subsequently the device for automatic adjustment of this flight time will be discussed in more detail below.

As already shown when considering the state of the art, it is particularly necessary to obtain consistently good impressions by always adjusting the flight times of the hammers so that they strike the print types offered by the rotating print medium carrier in the assigned print positions at the right moment.

The flight times result from the time span between the triggering time determined by the trigger circuit 7 and the point of impact on the print type 6 or the stop rail 1. The measurement or adjustment of the flight time can thus be carried out by shifting the triggering time. Instead of or together with the trigger time, other parameters can of course also be corrected, such as, for example, the duration or the level of the trigger pulse which is delivered by the trigger circuit 7.

The trigger time can be taken directly from the trigger signal.

The time of impact is determined by a stop detector 5 permanently installed on the stop rail 1. This stop detector 5 always emits a stop signal when a print hammer strikes the test type 23 which does not produce an impression on the paper web 4. The assignment of the stop signal to a print or hammer position is unambiguous, since only one print hammer is triggered during the measurement or adjustment of the hammer flight time during a test cycle, i.e. All print hammers in order according to the test type that passes the assigned print positions.

The stop detector 5 is designed in such a way that it responds to the shock wave that occurs in the stop rail 1 each time a pressure hammer strikes test type 23. The mechanical shock wave can generally be registered in three ways, and accordingly three basic types of impact detectors 5 can be used. These are electrical, optical or mechanical impact detectors. A large number of such impact detectors are known. Because of the technology already in the printer, an electrical method is preferable to the other options. With the electrical method, strain gauges, piezoelectric sensors and capacitive sensors can be used as stop detectors. Strain gauges are available in large numbers and are, for example, already integrated and adjusted together with the associated bridge circuit on foils, so that they can easily be attached to the stop rail 1. Piezo crystals are used in a similar way, where the required signal amplification is lower than with strain gauges. A capacitive sensor consists of two plates, one of which is an integral part of the stop rail 1 and, for example, is arranged on one end face. The second plate is isolated from the first plate, fixed with respect to the original position and parallel to this first plate at a very short distance. Each mechanical shock wave in the pressure rail 1, which is thus triggered by a stop of a pressure hammer 6 on the test type 23, changes the distance between the two plates of the capacitor and thus its capacitance. This change can either cause a voltage change by bringing the held plate to a higher potential via a circuit with a high impedance, so that rapid discharge is not possible, or the capacitor with its capacitance can be made to act as part of a tuned resonant circuit .

The stop signal supplied by the stop detector 5 is fed to an electronic evaluation circuit 8, to which the trigger signals supplied by the trigger circuits 7 are also fed. The Electronic evaluation circuit 8 determines the print hammer flight time from the time difference between the stop signal and the associated trigger signal and transfers this data to a downstream print hammer flight time memory 9. The output of the print hammer flight time memory 9 is connected via a microprocessor 10 to the inputs of a trigger time memory 11. The output of this memory is connected to the printer electronics 12 already mentioned, the output signals of which control the trigger circuits 7.

It should be pointed out that the hammer flight times are not about maintaining an absolutely always valid, predetermined value, since this value is due to other changing operating parameters of the printer - e.g. Paper thickness setting, printing energy limit. Trip coil temperature - can not give. It is important, on the other hand, that the times of the strokes do not deviate from the times defined in the time diagram of the printing process.

When the printer is switched on, the operator will initially set the triggering times manually in such a way that, if possible, all types of print are hit in the center. The hammer flight time measured at this point in time is stored as the average of all print hammers arranged in the printing station. Individual print hammers, the flight time of which deviates from this mean value by a certain amount, are adapted to the other print hammers with regard to the time of the stop by shifting the triggering time.

A hammer flight time adjustment in the form of a continuous automatic corrector will then proceed as follows. A test cycle is triggered at certain predetermined time intervals, for example, at the beginning or end of a form, the printing of an invisible line is inserted using the proposed large-area test type. The flight time measured for each individual print hammer is measured via the electronic evaluation circuit 8 and placed in the print hammer flight time memory 9. The microprocessor 10 then forms the mean value from it, detects deviations that exceed a permitted level, and calculates new triggering times for the individual print hammers. The triggering times of all print hammers are stored in the triggering time memory 11, from which they are made available to the printer electronics 12 controlling the triggering circuits 7.

The device according to the invention makes it possible to do without any mechanical adjustment of the print hammers. The automatic measurement and adjustment of the hammer flight times leaves no visible prints and can be done at any time with the record carrier inserted. The print hammers, which are only used sporadically during printing, are constantly checked and, at the time of their release, are always up to date with the printer's current status, e.g. adapted to the changed temperature. This would not be the case if the hammer flight time could only be measured while printing visible prints.

The time-of-flight monitoring device, which constantly takes all pressure hammers into account, makes it possible to dispense with very complex temperature compensation circuits or constant current sources which normally have to be used.

Claims (10)

1. Device for monitoring the flight time of the print hammers (6) of impact printers, where the printing process is effected by movement and impact of a print hammer (6) against a passing print type carrier (20) and an impact platen (1), the flight time being determined by a measuring device controlled via control signals derived at the moment of firing and impact of the respectively controlled print hammer (6), characterized in that apart from the standard print types (21) an additional test type (23) is provided on the print type carrier, the impact surface of said additional test type being of such a size that owing to the low surface pressure there will be no imprint on the record carrier (4) upon a print hammer impact, and that the flight time of a print hammer is determined upon its impact on the test type.

2. Device as claimed in claim 1, characterized in that the test type (23) consists of a surface projecting in accordance with the print types (21).

3. Device as claimed in claim 2, characterized in that the test type (23) is not connected with its entire surface with the print type carrier (20) so that the flexibility of the latter remains unchanged.

4. Device as claimed in any one of claims 1 to 3, characterized in that the control signal controlling the firing moment is the firing signal controlling the print hammer (6).

5. Device as claimed in any one of claims 1 to 4, characterized in that the control signal characterizing the moment of impact of the print hammer (6) is derived from the mechanical shock wave released in the impact platen (1), and that a corresponding impact detector (5) is arranged at the impact platen (1

6. Device as claimed in claim 5, characterized in that a mechanically, optically or electrically acting transducer responding to shock waves is used as an impact detector (5).

7. Device as claimed in claim 6, characterized in that a strain measuring stripe is used as an impact detector (5).

8. Device as claimed in claim 6, characterized in that a piezo crystal is used as an impact detector (5).

9. Device as claimed in any one of claims 1 to 8, characterized in that it is used for the automatic flight time adjustment (Fig. 2) in that in predetermined intervals a test cycle is performed for all print hammers (6) by means of the test type (23) producing no imprint, that the flight time measured for each individual print hammer (6) is stored in a print hammer flight time storage (9), that a microprocessor (10) forms a mean value of the flight times, detects deviations exceeding a predetermined degree and calculates new moments of firing for the respective flight hammers (6), and that these moments of firing are communicated via a firing time storage (11) to print electronics (12) controlling the firing circuits (7) firing the print hammers (6).

10. Device as claimed in claim 9, characterized in that the impact signal supplied by the impact detector (5) is applied to an electronic evaluator circuit (8) which also receives the firing signals from the firing circuits (7), and which from these signals derives the flight time for each print hammer (6).

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