JPH0641208B2 - Inkjet recording device - Google Patents

Description

TECHNICAL FIELD The present invention relates to an inkjet recording apparatus, and more particularly to color inkjet recording, and more particularly to an inkjet recording apparatus adapted to obtain a high quality image by multi-valued image recording.

[Conventional technology]

Conventionally, in an inkjet recording apparatus, in order to improve image quality, it has been performed to reduce ink droplets to increase dot density. However, when the dot density is increased, the openings of individual nozzles become smaller and the nozzles are easily clogged. It has been considered that the dot density is at most 400 to 500 DPI because the recording head becomes difficult to manufacture due to the arrangement of many nozzles and the printing speed becomes slow.

However, recently, there has been a demand for obtaining higher quality images, especially in color recording.
In order to reproduce a more faithful original image and to obtain an image comparable to a silver salt record, further improvement in image quality is required.

In the conventional ink jet recording apparatus, the ejection amount is changed by changing the magnitude of the voltage applied to the ejection unit or the application time thereof according to the level of the digital image signal.

However, in order to change the magnitude of the voltage, the circuit for converting the digital image signal into the magnitude of the voltage according to the level is complicated, and time-division driving is difficult especially in a recording head having a large number of nozzles. On the other hand, it is relatively easy to change the voltage application time according to the level of the digital image signal.

That is, in the former case, for example, the minimum voltage at which ink is ejected from the nozzle is V _th , and the voltage at which the maximum ink droplet is ejected is V _th .
_{Let M be} the number of levels of the digital image signal, and _let L be the density level of the digital image signal in the recording head. When a voltage represented by (0 ≦ n ≦ L) is applied, the amount of ink ejected can be changed in n + 1 steps according to this voltage. However, in the case of such means, in order to drive the head, The frequency must be at least L times that of the binary value, and the process becomes troublesome when the dots have a screen angle.

Further, as a different example similar to the latter, instead of supplying a single pulse voltage as shown in FIG. 5 (C) to each pixel clock pulse (shown in FIG. )
The voltage of the high frequency pulse as shown in is supplied, and one small droplet of ink is ejected corresponding to one of the high frequency pulses,
US Pat. Nos. 4,353,079 and USP4,5, US Pat. Nos. 4,353,079 and USP4,5 have a method of producing a density gradation of one point by ejecting several small droplets.
No. 13,299, etc.

[Problems to be solved by the invention]

However, even if the array number of high-frequency pulses is changed according to the density gradation of dots as in the above-mentioned US patent,
Problems remain in the process of converting the digital image signal into the voltage application time.

An object of the present invention is to focus on the fact that it is relatively easy to change the voltage application time according to the level of a digital image signal, and to eliminate the drawbacks of the conventional example, an inkjet recording apparatus that can obtain a high-quality image. Another object of the present invention is to provide an inkjet recording apparatus suitable for obtaining a multi-valued high-quality image of color.

[Means for solving problems]

In order to achieve such an object, the present invention changes the application time of the recording signal applied to the ink ejecting means in accordance with the level of the digital image signal so that the density gradation can be obtained by the change of the ejected ink amount. In the inkjet recording apparatus described above, a conversion unit that converts the digital image signal into an analog pixel signal at a predetermined cycle, a pattern signal generation unit that generates a fixed pattern signal in synchronization with the conversion of the analog pixel signal, Modulation signal generating means for generating a modulation signal having a pulse width corresponding to the pattern signal with respect to the analog pixel signal, and application for applying the recording signal to the ink ejecting means for a time defined based on the pulse width of the modulation signal And means.

[Action]

According to the ink jet recording apparatus of the present invention, in performing the image processing, the digital image signal is converted into an analog pixel signal at a predetermined cycle, and the pattern signal generated from the pattern signal generating means in synchronization with the analog pixel signal and the analog signal. A modulation signal having a pulse width modulated by the pixel signal is generated, and a recording signal is applied to the ink ejecting means by the applying means for a time defined based on the pulse width of the modulation signal, so that a high-frequency multivalue is obtained. A head drive signal can be generated.

〔Example〕

Hereinafter, embodiments of the present invention will be described in detail and specifically with reference to the drawings.

FIG. 1 shows an embodiment of the present invention, in which 1 is driven by a motor (not shown) to convey a recording material such as paper,
A platen roller that forms and holds a recording surface, and conveys a recording material in the form of continuous paper or cut paper while being held by a pinch roller (not shown) between the recording head 3 and the platen roller 1. The recording head 3 is mounted on a carriage 4, is driven in the S direction in the figure along two guide rails 5A and 5B, and recording is performed on a recording material in the process. The carriage 4 is connected to a belt 7 stretched between the pulleys 6A and 6B.
It is driven by a motor 8 provided coaxially with.

Further, the recording head 3 in this example is, for example, yellow (Y),
It has four recording units corresponding to four colors of magenta (M), cyan (C), and black (B), and these recording units have gear pumps 9Y, 9M, 9C, and
Supply pipes (Y) 10Y, 10M, 10 arranged with 9B and 9B
C and 10B are connected, ink tanks 11Y, 11M, 11
Ink is supplied from C and 11B. Separately from the above, each of these units is connected to the corresponding ink tank by a pipe without a gear pump in the middle, but in this example, only the pile 12Y that supplies yellow ink for simplification. Illustrated.

Reference numeral 13 denotes a cap portion that is provided at a position facing the ejection portion of the head 3 at the home position of the recording head 3 and that can move in the f direction and be joined to the head 3.

FIG. 2 shows an example of the configuration of each recording unit 30 that constitutes the recording head 3. In the figure, 31 is a substrate formed of a material such as aluminum or SiC, on which a base layer 32 made of Si is formed.
Are stacked. This base layer 32 is cut out from a Si wafer, and a nozzle portion is formed on this.

On the base layer 32, a heat storage layer 33 for holding heat generated from the electrothermal conversion element (heater) 34 driven by an image signal supplied from the control unit for a predetermined time is formed of, for example, SiO ₂ . . A heater 34 made of, for example, HfB ₂ is disposed in the ink flow path 35 formed on the surface of the heat storage layer 33, and is connected to the control unit via an electrode such as aluminum (not shown). The heater 34 is energized for about 5 to 20 μs for each pulse of the image signal, which causes the ink flow path.
The ink liquid in 35 is rapidly heated and vaporized, and
Ink drops are ejected from 36. Although not shown, the surface of the heater 34 has electrical insulation, heat resistance, liquid resistance,
It is coated with a material such as SiO ₂ and Ta, which has excellent cavitation resistance.

The partition wall 37 and ceiling plate 38 that limit the ink flow path 35 can be formed by exposing a dry film, and these are an adhesive layer.
It is joined through 39. Further, the ink is supplied to the ink flow path 35 through the liquid chamber 40, and the above-mentioned pipe is connected to the liquid chamber 40.

With the ejection ports 36 of the recording unit 30 thus configured aligned in the vertical direction in FIG. 1, four recording heads 3 are mounted and fixed on the carriage 4 corresponding to the ink colors.

In the recording head as shown in FIG. 2, when the heater 34 is energized, bubbles are generated and ink is ejected in the process as shown in FIGS. 3A and 3B. It is to be noted that here indicates a time course, and when a voltage is applied to the heater 34 through the electrode 41, the surface temperature T of the heater 34 rapidly rises as shown in FIG. The volume B of 42 expands with a slight delay. Due to this expansion, the pressure P in the nozzle increases, and the ink 43 in the nozzle is ejected as small droplets 44. Further, the pressure rapidly drops in the nozzle due to the contraction and disappearance of the bubbles, and the ink 43 flows from the liquid chamber toward the nozzle. The size of the ejected ink droplet is 15
In the case of ~ 30 μm, it becomes a droplet of about 30-50 μmφ.

In this case, the appropriate value of the applied voltage (V _op ) is
When the voltage value discharged from the nozzle is V _th , V _th is 1.
About 1 to 1.7 times is practical, and the application time t of the applied current is about 5 to 20 μs, and the pixel clock frequency at this time is about 0.5 to 10 KHz.

In the above, the case of binary recording in which ink droplets are ejected or not ejected is described, but the basic operation in the case of performing multi-valued recording will be described next.

That is, in the present embodiment, the recording head as shown in FIG. 2 is used, and a high frequency signal as shown in FIG. 4 is supplied to the heater to drive the head. Where that
(A) shows the pixel clock signal, and (B) shows the drive signal in the case of binary recording, but the difference from binary recording is that the minimum time of ink droplet ejection is t _th and it matches the recording density. _Assuming that the time to eject an appropriate ink droplet t _th is t _M , the time of (t _M −t _th ) is divided according to the density level of the digital signal, and the time is divided into the recording head as shown in (C) of FIG. This is the point of supplying a high-frequency drive signal. Note that V _op represents the applied voltage.

By adopting such a method, it is possible to change the ejection amount of the ink ejected from the recording head according to the application time of the high frequency pulse, and the details thereof are described in the above-mentioned US patents and the like. However, it is roughly as follows.

That is, when a pulse as shown in (C) of FIG. 4 is applied, minute ink droplets are ejected according to one cycle of the pulse, or coalesce with larger ink droplets ejected earlier during ejection. The driving voltage V _op in this case is slightly higher than that of the binary driving signal shown in FIG. 4B, but the high frequency is
It is about 100 KHz to 1 MHz. In this case, the pixel clock frequency used is 300 Hz to 5 KHz.

Therefore, the present invention uses the high-frequency signal as described above in order to carry out the multi-valued recording, and how to further add the digital image signal to the recording head efficiently in the application time of the voltage applied to the recording head. In order to solve the problem of whether to convert, the configuration of the device relating to the image processing will be described below.

In FIG. 6, reference numeral 101 denotes a digital data output device, which performs color conversion and A / D conversion of image data from a CCD color sensor or video camera (not shown) in the device 101, and further converts each pixel data into density information of a predetermined bit. Is converted into digital data, and each color component is output separately.
This digital data may be stored in the memory or may be input from an external device by communication or the like.

When the color printing section sequentially performs printing for each color, as shown in FIG.
One of M, C, and BK signals is selected, input to the digital-analog converter (D / A converter) 103, and converted into an analog quantity. Thus, the converted analog pixel signal 103S is sequentially input to one terminal of the comparison circuit 104.

On the other hand, the pattern signal generator 105 outputs a triangular wave pattern signal 105S as shown in FIG. A at a rate of once for every predetermined pixel of the digital data output from the digital data output device 101, and the comparison circuit 104 outputs the triangular wave pattern signal 105S. Input to the other terminal. 106 is a horizontal synchronizing signal generating circuit, 107 is an oscillator for generating a reference clock signal 107S, 108 is a timing signal generating circuit, and the clock signal 107S from the oscillator 107 is converted into a horizontal synchronizing signal 106S from the horizontal synchronizing signal generating circuit 106. In synchronization, the pixel clock signal 109S is counted down to, for example, 1/4 cycle and the digital data output device 101 and
The screen clock signal 110S is output to the D / A converter 103 and to the selector 111. Here, 112 is a counter provided for selecting the delay line 113 via the selector 111 by the horizontal synchronizing signal 106S.

The pixel clock signal 109S is used for the transfer clock of the digital data and the latch timing of the D / A converter 103. The horizontal synchronizing signal 106S may be generated internally or may be given from the outside. Thus, the comparison circuit 104 compares the signal level of the analog-converted pixel signal 103S with the signal level of the pattern signal 105S, and the pulse-width modulated multi-valued image data signal 10
4S is output, and the high-frequency pulse signal 114S generated by the oscillator 114 is supplied to the switching circuit 115 in accordance with the pulse-width-modulated multilevel image data signal 104S.
It is possible to supply the multi-valued head drive signal 116S to the ink jet recording head by turning it on and off in (1).

The above operation is performed for each of the Y, M, C and BK signals by supplying the select signal 117S from the color select signal output means 117 to the selector 111, and the delay line 113 is selected according to the timing. Then, the head drive signal 116S is output in the same procedure, and thus a full-color print can be obtained by overlapping the colors.

Next, the timing of each signal will be described with reference to FIG.

Here, the pixel clock signal 109S is the timing generation circuit 10S.
The signal obtained by counting down the reference clock signal 107S at 8 is output in synchronization with the horizontal synchronizing signal 106S. The pixel clock signal 109S is input to the D / A converter 103 and the digital data output device 101 and used as a pixel data transfer clock. Screen clock signal 110S
Is a signal obtained by further counting down the pixel clock 109S obtained by the timing generation circuit 108 by the timing generation circuit 108.
110S is, for example, a synchronizing signal for generating a triangular wave pattern signal 105S, and a signal line 120 (no delay) in the case of black and white via the selector 111 to the pattern signal generator 105.
Entered by.

As is apparent from FIG. 7, the analog pixel signal 103S and the pattern signal 105S are compared by the comparator 104, and when the pattern signal 105S is larger than the analog pixel signal 103S, it is set to 1 and when it is smaller, it is set to 0 and binarized. The image data signal 104S is output.

The pattern signal 105S input for comparison with the analog pixel signal 103S is determined as follows.

In FIG. 8C, assuming that the image signal level is in the range of m to M, the corresponding pulse widths W ₁ and W ₂ are determined, but W ₁ is almost the same for ejecting ink from the nozzle. The minimum time width, W _2, is the pulse width corresponding to the time required to generate the largest ink droplet that fits the recording density. Therefore, the cycle of the triangular wave of the pattern signal 105S is, for example, W ₂ (B) so that the image density level m → M can be relatively large.
It is recommended to match with one cycle of the screen clock signal 110S shown in (1) or to set the level M close to zero.

Therefore, as shown in (C) of FIG. 8, a triangular wave which is the source of the pattern signal is generated in the cycle of the screen clock signal 110S. Among them, the pixel clock signal 10 shown in (A) is generated.
Filter only the waveform that matches the rising edge of 9S (solid line in the figure) and do not use many triangular waves (dashed line in the figure).

In the above description, the color inkjet apparatus has been mainly described, but as described above, the present invention also applies to the monochrome monochrome inkjet apparatus.

Further, as the inkjet device, an inkjet device that ejects ink by using thermal energy, especially when a high-frequency pulse current is applied has been described, but an inkjet device that converts thermal energy into mechanical energy is also applicable. Of course, the present invention can be applied as long as the ink ejection amount changes in accordance with the current supply time, and similarly, a high quality image can be obtained.

Furthermore, the present invention can be applied to an ink jet recording means using a device such as a piezo device that converts electric energy into mechanical energy under the following conditions.

That is, any element can be used as long as the ejection changes in proportion to the application time of the voltage applied to the head, and the present invention is effective in the case of using such a recording head, and a high quality image can be obtained.

〔The invention's effect〕

As described above, according to the present invention, a conversion means for converting the digital image signal into an analog pixel signal at a predetermined cycle, and a pattern for generating a fixed pattern signal in synchronization with the conversion of the analog pixel signal. Signal generating means,
Modulation signal generating means for generating a modulation signal having a pulse width corresponding to the pattern signal with respect to the analog pixel signal;
An ink jetting device applies the recording signal to the ink ejecting device for a time defined based on the pulse width of the modulation signal, converts a digital image signal into an analog pixel signal, and generates a pattern in synchronization with the analog pixel signal. Since the multi-valued image signal of which pulse width is modulated is generated from the fixed pattern signal generated from the signal generating means and the analog pixel signal, in order to generate the ejection signal for obtaining the density gradation. It is possible to provide an inkjet recording apparatus suitable for carrying out color recording, because it is possible to obtain a high-definition and fine image without the need for providing a complicated structure.

[Brief description of drawings]

FIG. 1 is a perspective view showing an example of an inkjet recording apparatus to which the present invention is applied, FIG. 2 is a perspective view of a recording head in an electric-thermal-mechanical energy conversion type inkjet recording apparatus, and FIGS. 3A and 3B are FIG. 2 is an explanatory diagram of an ink ejection operation by the recording head, FIG. 4 is an explanatory diagram of a multilevel driving method, FIG. 5 is an explanatory diagram of a high frequency pulse used for ink ejection, and FIG. 6 is the present invention. FIG. 7 is a configuration diagram of such an image processing circuit, FIG. 7 is a timing chart of each electric signal in the circuit shown in FIG. 6, and FIG. 8 is an explanatory diagram for generating a pattern signal according to the present invention. 3 ... recording head, 101 ... digital data output device, 103 ... D / A converter, 103S ... analog pixel signal, 104 ... comparator, 104S ... multi-valued image data signal, 105 ... pattern Signal generator, 105S ... pattern signal, 106 ... horizontal synchronization signal generation circuit, 108 ... timing signal generation circuit, 110S ... screen clock signal, 115 ... switching circuit, 116S ... head drive signal.

Claims (4)

[Claims] 1. An application time of a recording signal applied to an ink ejecting means is changed according to a level of a digital image signal,
In an inkjet recording apparatus capable of obtaining a density gradation by changing the amount of ejected ink, a conversion unit that converts the digital image signal into an analog pixel signal at a predetermined cycle, and a conversion unit that synchronizes with the conversion of the analog pixel signal. Pattern signal generating means for generating a fixed pattern signal, and a modulation signal generating means for generating a modulation signal having a pulse width corresponding to the pattern signal for the analog pixel signal,
An ink jet recording apparatus comprising: an applying unit that applies the recording signal to the ink ejecting unit for a time defined based on the pulse width of the modulation signal. 2. The ink jet recording apparatus according to claim 1, wherein the applying means applies a signal obtained by switching a high frequency pulse signal by the modulation signal to the ink ejecting means as the recording signal. Inkjet recording device. 3. The ink jet recording apparatus according to claim 1 or 2, wherein the ink ejecting means has a heating element, and the heating element is a means for generating bubbles. Inkjet recording device. 4. The ink jet recording apparatus according to claim 1 or 2, wherein the ink ejecting means has an electrostrictive element, and ejects ink by mechanical strain of the electrostrictive element. An ink jet recording apparatus characterized by being present.