EP0835757B1 - Method of driving the piezoelectric elements in a print head of a droplets generator - Google Patents

Description

WO 95/25011 describes a method for operating a print head of an ink jet printer known. The printhead has one Large number of channels arranged side by side, each with a nozzle assigned. By activating a channel, the corresponding one is Ejected a droplet of ink. With control impulses is achieved that pressure waves within an activated Decay faster. With this solution, the Amplitude values of the pulses set, for which linear amplifiers are needed. These have poor efficiency and require a complex circuit. Limit the pulse widths to integer multiples of an acoustic period L / c, where L is the channel length and c is the speed of sound in the liquid is. Due to the complexity of the impulses it is only possible all channels with the same control voltage and to operate with the same pulse width.

Another operating method for is known from US-A-5 461 403 known a piezoelectric print head. The width of the Control pulses are varied to determine the drop speed and to modulate the drop volume. So different Grayscale are generated. A variation in the pulse width leads to change the drop size. The numerous values the pulse parameters require extensive tabulation. Due to the complexity of the table, it is only possible to use all channels with the same control voltage and pulse width to operate.

With both previously known solutions, there can be an impairment of the print image come when the print head with constant Relative speed compared to the paper to be printed becomes. The present invention is based on the object to provide a method of operating a printhead which avoids the above disadvantage. This task is accomplished by the Combination of features of the claims solved.

Figur 1

einen schematischen Längsschnitt durch einen Druckkopf mit einem Blockschaltbild der Ansteuerung,

Figur 2

einen Horizontalschnitt durch den Druckkopf,

Figur 3

einen Querschnitt,

Figuren 4 und 5

Kennlinienverläufe der Steuerimpulse,

Figur 6

drei verschiedene Impulsformen,

Figur 7

ein Blockschaltbild einer integrierten Ansteuerung,

Figur 8

eine Schaltung für die Gruppenauswahl,

Figur 9

ein Ausführungsbeispiel einer Logikschaltung zur Auswahl einer Impulsform,

Figur 10

ein Ausführungsbeispiel einer Logikschaltung für mehrere Kanäle, und

Figur 11

das Abklingen der Druckwellen in benachbarten Kanälen.

An exemplary embodiment of the invention is explained below with reference to the drawings. It shows:

Figure 1

1 shows a schematic longitudinal section through a print head with a block diagram of the control,

Figure 2

a horizontal section through the printhead,

Figure 3

a cross section,

Figures 4 and 5

Characteristic curves of the control impulses,

Figure 6

three different pulse shapes,

Figure 7

a block diagram of an integrated control,

Figure 8

a circuit for group selection,

Figure 9

an embodiment of a logic circuit for selecting a pulse shape,

Figure 10

an embodiment of a logic circuit for multiple channels, and

Figure 11

the decay of the pressure waves in neighboring channels.

In Figures 1 to 3 is part of a piezoelectric schematic Printhead 1 of an ink jet printer based on the shear converter principle greatly enlarged and not shown to scale. It consists of a piezoceramic plate 2, in which side by side a variety of longitudinal, identical, in cross section rectangular channels 3 is incorporated and one Cover plate 4 and a nozzle plate 5, which at one end each channel 3 has a nozzle 6. On the opposite All channels 3 are at the front end with one another via a transverse channel 7 connected in the cover plate 4. A connecting line opens into channel 7 8 to an ink reservoir 9. Each partition 10 between the channels 3 is on both sides on a partial area with an electrode 11, ie an electrically conductive layer Mistake. The plate 2 is mounted on a base plate 12. An electrical voltage is applied to the pair of electrodes of a wall 10 applied, so arises due to the direction of polarization of the piezo material, a shear of the channel partition 10. The clamping deforms the wall 10 as in FIG. 3 is outlined. If two adjacent walls 10 are deformed in opposite directions, so there is an increase or decrease in volume of the activated channel 3a. The applied to the electrodes 11 Pulse shape becomes an intake pulse and an opposite one Ejection pulse divided. They deform during the intake pulse Walls of the activated channel 3a as shown in Figure 3, see above that ink is drawn from channel 7 into activated channel 3a becomes. When the ejection pulse occurs, the activated walls 10 become in opposite directions deformed so that a droplet from the nozzle 6 of the activated Channel is ejected.

As can be seen from Figure 3, are shown in the Scherwandler type when activating one channel 3a also the influenced directly adjacent channels 3b. The pulse shape is chosen so that the resulting one Pressure vibration in these adjacent channels 3b is not sufficient to eject a droplet from their nozzles. With the described However, the converter type should not coincide with the activated one Walls 10 of the channel 3a one of the immediately adjacent walls 10 can also be activated because otherwise the pressure fluctuations in Channel 3b would be too large. That is why it is with this type of converter expedient to operate the channels 3 and thus the nozzles 6 so that only every third channel is activated at the same time becomes. The channels and their control are therefore in Divided into groups of three, which are operated one after the other. However, the channels can also be in groups of four, five or six which are operated one after the other.

Because of the connection channel 7 are now when activating one Channel 3a not only the immediately adjacent channels 3b, but also more distant channels through the emerging Pressure vibration affected. The inventors have found that the ejection speed of the droplets from an activated Channel 3a is different with constant pulse shape, depending on whether at the same time with this one channel 3a none or a third adjacent channel 3c or both third adjacent channels 3c to be activated. This difference in drop speed is disadvantageous because it adversely affects the printed image. It can be changed by changing the pulse shape depending on the number of the simultaneously activated third adjacent channels avoided become.

In Figure 4 is, for example, that for a constant drop speed of v = 6m / s required voltage for the suction pulse plotted as a function of the pulse duration. As from Figure 4 can be seen, the pulse shape by changing the applied Voltage and / or adjusted by changing the pulse width t1 be that the drop speed regardless of the number of simultaneously activated third adjacent channels is constant is. Because of the simpler circuit, the adjustment preferred only the pulse width. As can be seen from FIG the minimum suction pulse height with none activated at the same time third adjacent channel 0.91 of the acoustic period. To with the same drive voltage the same ejection speed with one or two simultaneously activated third adjacent channels to achieve a pulse width of 1.23 or 1.33 der acoustic period required.

FIG. 5 shows a similar diagram for the ejection pulse t2, the pulse width on the time axis in turn as a multiple the acoustic period and on the ordinate the refill time are plotted as multiples of the acoustic period. The pulse voltage is adjusted in such a way that in turn a constant Drop speed of 6m / s is achieved. The refill time is the amount of time it takes for the fluid meniscus reached its starting position at the nozzle 6 again Has. Again, the three variants are plotted on which at the same time with the activated channel no third Neighbor, a third neighbor or two third neighbors activated become. The curves found have several intersection points. It is therefore possible to operate on one of these intersections get by with just a single output pulse shape. Optimal is the intersection at which the refill time is minimal is. This is 1.1 times the acoustic period of the Case.

Figure 6 shows the three determined pulse shapes for operation with none (Fig. 6a), one (Fig. 6b) and two activated at the same time third adjacent channels (Fig. 6c). The suction impulses 13 different pulse widths and the shape of the Ejection pulses 14 are constant.

As can be seen from Figure 3, are the outermost two channels 3d of the printhead cannot be activated because of their outer wall is rigid. For example, 64 could be activated in the print head Channels, so he has a total of 66 or 68 channels, whereby the outermost n channels are unused. A printhead with 64 channels that can be activated requires 65 piezo actuators and 66 electrical connections. The outer wall of the outermost Channels 3d act for the pressure oscillation in the transverse channel 7 like a mirror. The reflection there took place in one the nearby operated channel has the same influence as if the mirrored one third or sixth adjacent channel operated simultaneously would. This is when assigning the intake pulse width appropriately considered this channel.

FIG. 1 schematically shows an integrated control circuit 15, which is conveniently attached to the base plate 12. Thereby becomes the number of lines used to control the printhead 1 are significantly reduced.

The function of the integrated control circuit is shown in FIG. 7 clarified. The block diagram shows the most important internal ones Sub-functions consisting of circuit breaker 16, selection logic 17 and shift register 18. For the electrical connection to Printer controls are used in this particular embodiment only 13 lines required. One advantage is that the number of the lines even with an increase in the number of channels and thus the number of converters remains constant. The power supply for the power and logic part is done via the connections POWER, PGND, VCC, and GND. Via a RESET connection the control is set to a defined basic state. The connections G1 to G4 and the connection NEXT are used for Control of drop generation, where G1 to G3 are the three different ones Intake pulse widths and G4 as the exhaust pulse width means. The connections DSERIN, DSEROUT and DCLK serve of data transmission, with the DSEROUT output for service purposes serves. The data block transferred to the shift register is sent back to the PC or the printer control and there with the data block transmitted via DSERIN compared. Thus a correct data transmission are checked. Furthermore, there is Ability to transfer status information from the printhead (Temperature too high, ink empty etc.) and evaluated on the PC. An entire data block is used for operation via DSERIN all 64 nozzles (in the aforementioned example) into the shift register read. The nozzles 6 are operated in three phases. In the data block So there is the information which nozzles in the next Phases are operated, i.e. the pattern to be printed.

FIG. 8 shows the first part of the selection logic 17. As soon as If a data block is read in, the NEXT signal activates the first phase Ph1 belonging nozzles, provided by the content of the shift register (the upper row of digits in Fig. 8) is selected are. The signals Ph1, Ph2 and Ph3 are used in succession the NEXT signals generated by the phase switch 22. The output signals on the output conductors 23, 24, 25 of the phase selection switch 22 are via AND gates 26 with the input signals linked from the shift register 18. This ensures that only at most every third channel of the printhead is activated at the same time. After Ph3 is with the next NEXT signal started again with Ph1. If at this time not already a new data block via a DCLK signal the DSERIN input has been read into the shift register 18 the three phases repeated, the nozzles 6 in the same pattern activated again. This allows different shades of gray be achieved. If no shades of gray are required, it follows Read in a new data block after every third NEXT pulse clocked into the shift register 18 via the input DSERIN by DCKL. As soon as the new data block has been read in, this can be done next pattern printed with a sequence of three NEXT pulses become. The data transmission and the NEXT pulses are through the printer hardware synchronized and in function of the movement the print head controlled relative to the paper to be printed.

The second part of the selection logic 17 is shown in FIG. 9. It shows an embodiment with one with simple logic gates built circuit for the selection of the pulse shape any channel i, depending on the adjacent channels. The The signal for channel i is at one of the three inputs of three AND gates 27 connected. The signal for the two third adjacent channels i-3 and i + 3 is connected to the other two inputs in the first gate 27 via an inverter 28 each second gate 27 via an EXCLUSIVE OR and to the third gate directly connected. Depending on whether one, one or both third adjacent channels i ± 1 when the signal for the channel is switched on i is activated at the same time, a signal appears t10, t11 or t12 on the first, second or third gate 27.

This selection circuit 30 is for all channels that can be activated 3 of the print head 1 is present, as shown in FIG. 10 is. Each of the three outputs t10, t11, t12 of the circuits 30 is via an AND gate 31 with the three lines 32, 33, 34 linked to which the three signals G1, G2 and G3 for the three different intake pulses 13 are pending. The exit of the three gates 31 assigned to a circuit 30 go to the input an OR gate 35. The pulse length at the outputs of the Gate 35 is then dimensioned so that the drop speed is independent of the number of thirds activated at the same time Adjacent channels. The circuit 36 according to FIG. 10 still follows known intrusion of the ejection pulses to the activated ones Channels (inputs at the top of Fig. 10), with which then the circuit breakers 16, the electrodes 11 are controlled.

The circuit shown is only one of many possible Embodiments that for the sake of simplicity was chosen. Logic functions can be done by any combination of gates can be realized, with simplifications are conceivable in which partial functions are already in others Function blocks can be realized, for example by double To avoid negations.

The solution according to the invention can be refined if in addition to the number of third adjacent channels, the number of the sixth adjacent channels activated at the same time (their influence on the exit speed is however lower) is taken into account. The circuitry is however higher and there are a total of nine different suction pulse shapes required, from each of which the applicable by a appropriate logic circuit is to be determined.

FIG. 11 shows a further possibility of refinement: The decay of the pressure waves in neighboring ones is shown Channels if channel 0 has been activated. As can be seen the pressure fluctuations in the first adjacent channel are relatively considerable and decrease with increasing channel spacing. Are the Pressure vibrations in a channel have not subsided before it is activated (for example in phase 2 or 3 in Fig. 8), so there are changed initial conditions due to this history, which also affects the drop speed effect. Especially with printheads, which are the phases follow quickly, i.e. quickly from a nozzle group the other is switched over, it is advisable to use the Selection of the pulse shape, in particular the pulse duration, in addition to take into account how many first and second adjacent channels in a fixed interval before the activated channel is triggered were operated.

The exemplary embodiment described is a Piezoelectric printhead of the shear transducer type. But there are other types of piezoelectric printheads are also possible Example those with a bending oscillator over each nozzle, for Example according to EP-A-713 773. With this type of converter also two neighboring nozzles are activated at the same time. Also at the present invention is applicable to these printheads because even with these via pressure vibrations when activating one Adjacent channels can be affected. In this case of course the link condition is different, so for example the number of simultaneously activated first and second adjacent channels can be taken into account.

Claims (9)

1. A method for controlling piezo-elements in a printhead (1) of a droplet generator with a multitude of adjacently arranged ink channels (3), where the piezo elements (10, 11) are controlled such that the exit velocity of the droplets is independent of the number of simultaneously activated neighboring channels (3c), wherein
   the form of the activation impulses is modified depending upon how many neighboring channels (3c) are simultaneously activated.
2. The method according to claim 1, wherein the impulse duration of the suction impulse (13) and/or the expulsion impulse (14) is varied.
3. The method according to claim 2, wherein no more than each nth channel (3) is activated and wherein three different impulse forms are used depending upon whether activation occurs in none, one or two nth neighboring channels (3c).
4. The method according to claim 2, wherein no more than each nth channel (3) is activated simultaneously and nine different impulse forms are used depending upon whether activation occurs in none, one or two nth neighboring channels and/or none, one or two 2nth neighboring channels.
5. The method according to one of claims 1 to 4, wherein the impulse form is varied depending upon how many first and second neighboring channels were activated at a fixed time interval prior to triggering of actual droplet expulsion.
6. The method according to one of claims 1 to 5, wherein the activation impulses comprise a suction impulse and an expulsion impulse, wherein the expulsion impulses are maintained constant.
7. The method according to claim 6, wherein the expulsion impulses are selected in a manner such that refilling time of channels is minimal.
8. The method according to one of claims 1 to 7, wherein an end of a suction impulse of each operated channel coincides with the beginning of an expulsion impulse of this channel.
9. The method according to one of claims 1 to 8, wherein on both sides of the printhead n channels are not operated, and wherein the last operated channel is operated in such a manner as if the non-existing 2nth neighboring channel were additionally operated.

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