DE3513442C2 - Method of operating an inkjet printer - Google Patents

Description

The invention relates to a method for operating a Inkjet printer according to the preamble of the claim 1.

The execution of practical inkjet printers that creating a single ink droplet on demand serve is relatively new in technology. With the known Inkjet printers with "drops on request or Ver long "is the volume of each individual ink droplet typically the geometry of the inkjet printer, the Type of ink used and the size of one in the inks Chamber for ejecting an ink droplet from an associated neten opening developed overpressure. The effective one Diameter and structure of the opening, the volume and the structure tur of the ink chamber associated with the opening, the transducer construction and the method of coupling the converter with the Ink chamber are other factors that affect the volume of the individual ink droplets ejected from the opening determine. In any such inkjet printer is for a high resolution image required that relatively small or low volume ink droplets be ejected by the printer. Typically, such Small size ink droplets by reducing the size Obtained diameter of the openings of the inkjet printer. However, it is difficult to make small-diameter jet openings manufacture knives, and the operation of an inkjet printer kers, the one with such a small diameter Openings is blocked by openings problems (due to dried ink, contamination in ink, paper dust, etc.) due to adverse effects due to a high ratio of surface chips  forces to inertia, by a poor result etc. negatively affected.

Many attempts have been made to reduce the print density as well as the print resolution or screening for an ink to control the jet printer. For example, in US Pat. No. 3,977,007 an ink jet printer reproduces gray shading by one determines the number of ink droplets on a predetermined Point position in a point matrix is selectively one changes.

In US Pat. No. 4,281,333, the volume or size that of a drop-on-demand ink jet printer "ejected ink droplets only by variation the amplitude or power envelope of the control signal wave lenform controlled to operate the inkjet printer is used. U.S. Patent 4,337,470 discloses that of a Ink jet printer created dot size by varying the Frequency of vibration of a vibrator controlled, which in ink vibrated in the ink head to Ejecting droplets of ink. These droplets become tax pressure density electrostatically on a receiving medium directed towards or away from this.  

A drip device is known from DE 27 23 037 A1, that works with electrostatic deflection. DE 27 23 037 A1 discloses various methods of controlling size of a resulting droplet. However, these procedures serve not to produce halftone representations, but to the action of this particular, on electrostatic Principles of operation of an inkjet printer to various Adapt paper types. Once set, the pressure remains intensity the same until it is adapted to a new type of paper becomes. For the generation of halftone images are those in DE-A-27 23 037 disclosed procedures unsuitable, because it would have to changed the printer setting for each pixel printed will. Even when printing text, the effort would be a lot high, because with changing intensity requirements of individuals The printer would have to reset words after each word will.

In the method according to claims 3 and 4 of DE 27 23 037 A1 becomes a certain generation rate of ink drop fen fixed. How many drops per pixel the paper then depends on how many in the for that Generate a pixel unit of time Sucked drops from the drop path electrostatically  will. The pixel intensity can therefore be regulated by for example the electrostatic charging time or the for the total time allowed to print a pixel or the Drop generation frequency changes. Change the drop generator However, frequency does not mean according to DE 27 23 037 A1 a change from pixel to pixel but a long term position depending on the type of paper to be printed. At the front Direction according to DE 27 23 037 A1 must namely to the transition to achieve a different pressure intensity, a variable resistor stood plugged a delay circuit, the Relativge speed between printhead and paper changed Ink flow speed regulated or by tampering with a character generator the data frequency can be set. All of these measures can be according to the in DE 27 23 037 A1 specified procedures only with a time expenditure reali sieren, the when inserting a different type of paper in the Printer or after moving the printer to another Work environment is justified, but not when switching from pixel to pixel or word to word can be performed.

A method of operating an inkjet printer to control the size of on a recording medium printed ink dots with an inkjet printer an ink chamber, one in communication with the ink chamber standing opening, one in communication with the ink chamber standing converter and a pulse source for generating Pulses with a drop generation frequency, the Transducer to the electrical impulses fills the ink chamber and ink droplets from an associated nozzle opening is known from EP 0 115 180 A2. EP 0 115 180 A2 is state of the art for the present invention Patent law § 3, paragraph 2. In the case of EP 0 115 180 A2 known method, a converter with pulse trains is determined ter time controlled so that the thereby during this period of time ejected ink droplets to a tin melt the drops before or when they drop the pressure reach dium for printing. Furthermore, from the  EP 0 115 180 A2 known to precede a dead time between the pulses hen.

The invention has for its object a method to operate an inkjet printer with which the size of a record in the simplest possible way Medium to be printed on dots is controllable to halftone generate images.

According to the invention, this object is achieved by a Method according to claim 1.

In the method according to the invention, the number of generated ink drops controlled per pixel and from pixel to Pixel depending on the requirement of the character to be printed or Image changed. The drop generation frequency remains constantly adjusted to the optimal dominant resonance frequency quenz. However, the drops are not generated continuously, but the amount of those with this optimal frequency per pixel drop to be generated is switched on and off of the Dimensioned drop generation mechanism. The advantage of the invent The method according to the invention is that the optimal trop generation frequency can be selected and no more Drops are generated as to deploy each desired pixels are actually necessary. Furthermore, one Change in intensity from pixel to pixel in reasonable time possible.

Advantageous developments of the Ver driving are the subject of claims 2 to 4.

The invention is described below with reference to the drawing explained in more detail. It shows:

Figure 1 shows an inkjet printer in a sectional view.

FIG. 2 shows a detail of the ink jet printer according to FIG. 1 in an enlarged view;

Fig. 3 is a projection view of the ink jet printer in an exploded arrangement, the ink jet printer comprises the exemplary embodiments shown in Figures 1 and 2;

Fig. 4 shows the waveform for electrical pulses of a preferred embodiment before;

5 shows a sinusoidal waveform for electrical signals of another embodiment of the invention.

Fig. 6 is a sine half-wave curve for a third embodiment of the invention;

Fig. 7 is a sine quarter wave curve for electrical pulses of a fourth embodiment of the invention;

Fig. 8 is a sawtooth waveform for a fifth embodiment of the invention;

Fig. 9 is a triangular waveform for electrical impulses se of a sixth embodiment of the invention;

Fig. 10 prints (A) to (F), which were obtained from the ink jet printer shown using the method according to the invention, and

Fig. 11 font printouts (A) to (C), which illustrate the typical print density control, which can be achieved by using the ink jet printer shown using the method according to the invention and

Fig. 12 droplets produced using the method according to the invention during the flight.

In Figs. 1 to 3, an inkjet printer is shown with which a printing operation has been achieved with high resolving power and high screening. The various embodiments of the invention, which are described below, can, however, be used in a large number of inkjet printers (in particular in inkjet printers with "drop-on-demand"). The ink jet printer explained below is therefore intended to explain the invention and is not limitative. Accordingly, only the basic mechanical features and operation of this printer are discussed in the following paragraphs.

The ink jet printer shown in FIGS. 1 through 3 includes a chamber 200 having an opening 202 for ejecting ink droplets in response to the excitation of a transducer, with a transducer 204 for each jet of a series of such jets (see FIG. 3) is provided. The transducer 204 extends along its axis of extension and contracts along its axis of extension (in the directions shown by the arrows in FIG. 2), this movement being coupled to the chamber 200 via a coupling device 206 . The coupling device 206 comprises a foot 207 , a viscoelastic material 208 adjoining the foot 207 and a membrane 210 which is prestressed into the position shown in FIGS. 1 and 2.

Ink flows from a non-pressurized container 212 into chamber 200 through a restricted inlet means in the form of a limited opening 214 . The limited opening 214 is formed by an opening provided in a flow restriction plate 216 (see FIG. 3). As can be seen from FIG. 2, the container 212 formed in a chamber plate 220 has a chamfered edge 222 which leads into the opening 214 . As can be seen from FIG. 3, the container 212 is equipped with an inlet pipe 223 and an outlet pipe 225 . The container 212 is flexible due to the membrane 210 . This membrane 210 communicates with the ink via a through opening provided in the flow restriction plate 216 , a large opening 227 which is adjacent to a recess 229 provided in the plate 226 .

One end of each transducer 204 is guided by the cooperation of the foot 207 with a 224 provided in a plate 226 . As shown, feet 207 of transducers 204 are slidably received in holes 224 . The other ends of transducers 204 are elastically secured in a block 228 using a resilient or resilient material 230 provided in slots 232 to provide support for the other ends of transducers 204 . Electrical contact with transducers 204 is also made in an elastic manner with the aid of a resilient printed circuit board 234 which can be connected by a suitable means, such as e.g. B. a solder is coupled to an electrode 260 of the transducer. Printed circuit boards 238 are provided on circuit board 234 .

The plate 226 (see FIGS. 1 and 3) has holes 224 at the base of a slot 237 which, as mentioned above, receive the feet 207 of the transducers 204 . The plate 226 is also equipped with a receptacle 239 for a heating intermediate layer 240 . This Heizzwischenlage 240 comprises a Hei zelement 242 with coils 244, a hold-down plate 246, a plate 246 of the associated spring 248, as well as immediacy bar below the heating element 242 disposed support plate 250th A slot 253 is also provided for receiving a thermistor 252 . Thermistor 252 is used to monitor the temperature of heater 242 . The entire heating intermediate layer 240 is held by a cover plate 254 in the receptacle 239 provided in the plate 226 .

As can be seen from Fig. 3, the various components of the ink jet printer described are held together by means of screws 256 which extend upwards through openings 257 and by means of screws 258 which extend downwards through openings 259 , whereby the screws ben 258 hold the circuit board 234 on the plate 228 in the correct place. The dashed lines in FIG. 1 show the connections 263 to the printed circuit 238 on the printed circuit board 234 . Connections 263 couple a controller 261 to the inkjet printer to control its operation.

In normal operation of the ink jet printer, controller 261 is programmed to supply voltage to a particular or some of heated electrodes 260 of transducers 204 at a convenient time via its connection to printed circuit 238 . The supplied voltage causes an electric field that is generated perpendicular to the axis of extension of the selected transducers 204 , causing the transducers 204 to contract along their elongated axis. When a particular transducer 204 contracts after excitation, the portion of the diaphragm 210 located under the foot 207 of the transducer 204 moves toward the contracting transducer 204 , effectively increasing the volume of the attached chamber 200 . As the volume of the particular chamber 200 expands, a vacuum is initially created in the chamber, causing the ink therein to tend to move away from the opening 202 , while at the same time ink from the container 212 through the associated restricted opening 214 can flow into the chamber 200 . The amount of ink flowing into the chamber 200 during refill is greater than the amount that flows back through the restricted opening 214 during the firing of the ink. The time between refilling and firing is not changed during blasting, thus providing a "refill before firing" cycle. The controller is programmed so that shortly thereafter the voltage or control signal is removed from a particular one or some of the selected transducers 204 , causing the transducer (s) 204 to expand very quickly along their elongated axes and via the visco-elastic material 208 and the Push feet 207 against the part of membrane 210 below them. In this way, a rapid contraction or reduction in the volume of the associated chamber or chambers 200 is achieved. This rapid reduction in volume of the attached chambers 200 generates a pressure pulse or an excess pressure disturbance in the chamber 200 , as a result of which an ink droplet is expelled from the associated openings 202 . It should be noted that with such excitation of a particular transducer 204, it both reduces its length and increases its thickness. However, the increase in its thickness for the illustrated inkjet printer is irrelevant insofar as the length changes of the converter control the use of the individual ink jets of the ink jet series. It should also be noted that the present technology, that is, by energizing the transducers for the purpose of contraction along their elongated axes, prevents accelerated aging of the transducers. In extreme cases, depolarization is also avoided.

As mentioned above, it is known that the droplets size produced by a pulse inkjet printer  becomes, closely to the size of the opening of the associated inks beam device is bound and that in general only slight variations in droplet size, e.g. B. by varying the control signal amplitude or control signal waveform, can be generated. However, for a high quality halftone print the droplet size in be controllable over a wide range.

In the operation of the ink jet device described above, it was recognized that one by synchronously energizing one or a combination of the fluidic and mechanical resonance frequencies of the ink jet printer to produce a dominant resonance frequency disturbance in the corresponding chamber and the corresponding ink and by Generating Las sen a unicyclic or a subharmonic Cycle of the dominant resonance frequency can control the volume of the ejected ink droplets. It has also been found that by repeating this operation in an iterative manner, with each subsequent cycle being in synchronism with the dominant resonant frequency of the ink jet printer, a plurality of ink droplets can be ejected within a period of time which allows the droplets to be released can fuse during flight through the air or on the recording medium. This enables substantial control over the dot size resulting on the recording medium relative to the dot size obtained from a single ink droplet. The resulting dot size depends on how often the method according to the invention for operating the printer is repeated within a given time period. Fig. 12 shows nine droplets 301 to 309 in flight to create a dot on a recording medium using the method of the present invention.

According to the invention it was also recognized that for the illustrative inkjet printer of this example the helmet holtz resonance frequency is the dominant resonance frequency of the actual inkjet printer. Other inks jet printer, also using the invention  Process can be operated as dominant Resonance frequency a different resonance frequency than the helmet have holtz resonance frequency. For the purpose of further Description and clarification of the Ver Driving to operate an inkjet printer is accepted men that the Helmholtz resonance frequency is the dominant Reso frequency is. However, such an assumption does not mean Limitation in terms of scope and ver application of the invention.

The present method is a multi-pulse method of operating an ink jet printer that uses the dominant resonant frequency of the ink jet printer to excite controllable volume ink droplets by pulsing transducer 204 (in this example) with a repetitive frequency of the dominant resonant frequency, with Ver application of a single pulse or a plurality of pulses depending on the required point size. Where the Helmholtz frequency is the dominant frequency, this frequency results from the interaction of the compliance of the ink chamber 200 (in this example) and the ink or fluid inertia, expressed by the formula

where C is the ink chamber compliance, L is the inertia, and 1 / L is, for example, [1 / L aperture 202 + 1 / L limited aperture 214 ].

Laboratory tests and analysis have shown that the inkjet printer shown has a Helmholtz frequency of approximately 30 kHz. To operate the ink jet printer shown according to the inventive method, as can be seen from Fig. 4, essentially rectangular or square wave pulses were used. The pulse characteristics of this particular waveform, which were found to provide substantial control over the size of the ejected ink droplet, were determined for the various time periods shown as follows: T ₁ = 1.0 microsecond pulse rise time, T ₂ = 13.0 Microsecond pulse duration, T ₃ = 1.0 microsecond pulse decay time and T ₄ = 15.0 microseconds dead time, whereby a pulse repetition rate near the 30 kHz Helmholtz resonance frequency of the printer shown is provided. It should be noted that in this example the dead time T _{4 is} required to make or couple the control signal supplied to a converter in phase with the natural vibration of the ink fluid contained in the ink chamber 200 . It was found that by supplying two pulses, as shown in Fig. 4, to a transducer 204, the volume of the final ink droplet ejected was approximately twice that obtained using a single pulse over the same time period in to which the two impulses were fed. It has also been found that the droplet volume appears to increase linearly in direct accordance with the number of such pulses applied to transducer 204 . By supplying two or more pulses of appropriate amplitude having the waveforms shown in Fig. 4 and the characteristics described above, it was further determined that this multi-pulse method results in the ink droplets fusing in flight or upon hitting the recording medium, from which there is an increased dot size on the recording medium compared to the use of a single pulse to produce such a dot on the medium.

It should be noted that the waveform of FIG. 4 and the waveforms of FIGS. 5 to 9 described below can be generated under laboratory test conditions with a commercially available wave generator. For example, in a practical printer, controller 261 must be specially designed or programmed to produce the desired waveforms and the number of pulses required to generate a particular spot size on the recording medium.

Tests performed have shown that the illustrated printer, which, as mentioned above, has a Helmholtz frequency of 30 kHz, using any combination of pulse duration T ₂ and dead time T ₄ , which is between 8.0 microseconds and 16.0 microseconds can move, work, the rise time T ₁ and the fall time T _{3 have been set} for example to one microsecond. The lower limit of this range is determined by the response time of the transducer or transducers 204 , whereas the upper limit of this range is determined by the construction of the inkjet printer, which determines the effectiveness of the control or operation of the printer with or in the vicinity of its Helmholtz Frequency limited. The complexity of the electronic structure of the control device 261 is reduced if the waveform of the control pulses, such as that in FIG. 4, essentially corresponds to that shown, the total pulse duration (T ₁ + T ₂ + T ₃ ) and the dead time T ₄ in are essentially the same. Optimal operation of the inkjet printer shown was also achieved if the total periodicity of the pulse train (T ₁ + T ₂ + T ₃ + T ₄ ) was made substantially equal to the reciprocal of the dominant resonance frequency, in this example 1 / F _H. It has also been found that due to the response time constraints of transducer 204 coupled with the relatively high frequency of the dominant resonant frequency mode of control and operation of the ink jet printer using the multi-pulse method of the present invention, many other different waveforms are different are as those of Fig. 4, but have the same or similar Periodi capacity can be used. Other waveforms found to provide satisfactory point size control using the method of the invention include, for example, a sine waveform, a half-sine waveform, a fourth-sine waveform, a sawtooth waveform, and a triangular waveform as shown in FIGS Fig FIG. 5 to 9 henfolge in this Rei. When using such alternative waveforms to operate the printer shown, the 30 kHz Helmholtz frequency of the printer was determined as the dominant frequency, as mentioned above. Accordingly, for the sinusoidal waveform of Fig. 5 1/2 T _{5 can be} made substantially equal to 30 kHz. Similarly, for the sine half-wave curve of FIG. 6, the pulse duration T ₆ and the dead time T _{7 should be} approximately 15 microseconds. Similar remarks apply to the pulse durations T ₈ , T ₁₀ and T ₁₂ of FIGS. 7 to 9 and the dead times T ₉ , T ₁₁ and T ₁₃ of FIGS. 7 to 9.

When testing various pulse or waveforms, it was found that the rectangular or square waveform with a much lower voltage pulse amplitude can be used than any other waveforms tested, such as e.g. As those of FIGS. 5 to 9, probably due to the faster rise and fall times. In fact, it was found that the quarter-wave sine waveform of FIG. 7 requires pulses of 20% greater amplitude than the substantially square or rectangular pulses of FIG. 4 in order to obtain equivalent printing from the illustrated ink jet printer. As noted above, in general, the waveform of FIG. 4 relative to the other waveforms of FIGS. 5 through 9 and relative to still other different waveforms can be provided much more electronically.

During testing of the method according to the invention and during operation of the inkjet printer shown, it was also discovered that due to the dominance of the Helmholtz frequency in the printer under test, the multi-pulse method according to the invention can also be provided by basing the periodicity of the control pulses on subharmonic cycles of the Helmholtz frequency. It is believed that the same result for the dominant resonant frequency would be obtained from some other ink jet printers if tested using the method of the invention. However, using the example of a 30 kHz main Helmholtz frequency in a special ink jet printer, a subharmonic frequency would result in control pulse widths that would be very large, which would result in an undesirable reduction in the usable print frequency of the special ink jet printer. Accordingly, the inventors tested an ink jet printer that is similar to the printer shown, but has a smaller ink chamber 200 (relatively low compliance) to provide a Helmholtz resonance frequency of about 100 kHz. With the aid of the method according to the invention, this printer could be operated with a satisfactory pressure, using multiple pulses with a periodicity of 30 microseconds, which corresponds to the third subharmonic of the 100 kHz Helmholtz resonance or main frequency. Multiple pulses that have a periodicity that has been made subharmonic to 100 kHz, e.g. B. of 20 kHz were tested, but the performance at this subharmonic level was found to be relatively poor.

As shown in Fig. 10, tapes of successive dots were printed by using a continuously larger number of multiple pulses to print each dot in the corresponding tapes of views (A) to (F). The multiple pulses used to generate the point bands in FIG. 10 were sine quarter waves according to FIG. 7, the pulse durations T ₈ and the dead times T ₉ each being 15 microseconds. The voltage amplitude of the pulses was kept constant at about 33 volts. In the point band of view (A), only such a pulse was used to achieve the points shown. The points of the band shown in view (B) were generated during the same cycle time as those in view (A), but instead of one pulse, two multiple pulses were used to generate each point of the band shown in view (B). Similarly, the dots of the tapes shown in views (C) through (F) were generated using 3, 4, 5 and 6 multiple pulses, respectively, using an equal time cycle for printing each dot. Accordingly, as expected, the bands of views (A) through (F) are progressively thicker, due to the progressively larger dot size obtained with the method according to the invention.

Similar multiple pulses were used to produce the font sets shown in views (A) through (C) of FIG. 11 with progressively greater fatness. The characters printed in view (A) required a control pulse to generate each of the dots that make up a particular character, whereas two pulses were used to generate each of the individual dots of the font of view (B) and three pulses to generate each of the individual dots that form the characters of view (C) were used.

The controller 261 can be used to perform the required control functions z. B. with a hardwired logic or with a programmed microprocessor or a combination nation of both. Was used to generate the waveforms shown in Figs. 4 to 9 of the countries OF INVENTION a Wavetek waveform generator, model 175, manufactured by Wavetek, San Diego, California, used. In a practical implementation of the printer, controller 261 would typically be designed to provide the required waveforms and functions, as discussed above, for each particular application.

Claims (4)

A method of operating an ink jet printer for controlling the size of ink dots printed on a recording medium, comprising an ink jet printer having an ink chamber ( 212 ), an opening ( 214 ) communicating with the ink chamber ( 212 ), one with the ink chamber ( 212 ) connected transducer ( 204 ), a pulse source ( 261 ) for generating pulses with a drop generation frequency, the transducer ( 204 ) filling the ink chamber ( 212 ) on the electrical pulses and ink droplets from an associated nozzle opening ( 202 ), characterized in that

• - A number of pulses depending on the required point size with a dominant resonance frequency or a subharmonic of the dominant resonance frequency of the ink jet printer are generated with a dead time (T4, T5, T7, T9, T11, T13) between the individual pulses and
• - The transducer is fed with the pulse sequences generated in this way, an ink droplet with a certain volume is ejected from the nozzle opening ( 202 ) on each of the electrical pulses and the ink droplets ejected during a pulse sequence flow together to form a single resulting ink drop before or when the ejected ink droplets reach the print medium for printing.

2. The method according to claim 1, characterized in that the period of the electrical impulses either essentially equal to the period of the Helmholtz resonance frequency or im essentially equal to the period of a subharmonic of the Helmholtz frequency is.   3. The method according to claim 2, characterized in that one shapes each electrical impulse so that it essentially a pulse with an exponential leading edge and represents a step-shaped trailing edge. 4. The method according to claim 1 or claim 2, characterized ge indicates that the electrical impulses essentially to a square wave, a rectangular one Wave, a triangular wave, a sine half wave form, a sine full waveform, a sine quarter wave lenform, a sine waveform with less than one Quarter wave or a pulse with an exponential Front flank and a step-shaped rear flank forms.