Abstract:
An inkjet recording device includes a nozzle module, a switching unit, a waveform generating unit, an image recognizing unit and a pulse width modulating unit. The image recognizing unit determines an ejection condition of the ink droplet ejected from the nozzle while referring to ejection data indicating a type of each pixel to be recorded, and generates switch pulse width data that includes the ejection data and the ejection condition. The pulse width modulating unit generates the switch pulse based on the switch pulse width data. The switching unit opens and closes in response to a switch pulse. An opening duration of the switch unit is variable depending on the switch pulse.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an on-demand type inkjet recording device, and particularly to a high-speed inkjet recording device that records images using a plurality of nozzles. 
   2. Description of Related Art 
   An inkjet recording device provided with a recording head having a plurality of nozzles can record images at a high rate of speed and at a high density on recording medium due to the plurality of nozzles. 
   Such inkjet recording devices are categorized as continuous type or on-demand type devices. The on-demand type inkjet recording device, such as that disclosed in Japanese unexamined patent application publication No. 2002-273890, has a simpler construction than that of the continuous system. Therefore it is possible to dispose hundreds or thousands of nozzles to be disposed at a high density in the on-demand type inkjet recording device. 
   However, in such a multi-nozzle inkjet recording device, the ejection velocity and weight of ink droplets ejected from multiple nozzles tend to vary widely among nozzles. When the ejection velocity varies, the position at which ink droplets land on the recording medium also varies, leading to an obvious deterioration in image quality in lines of text, figures, tables, and the like. When the weight of the ink droplets varies, on the other hand, the surface area of the dots on the recording medium also varies, producing irregular densities in the image, particularly halftone images. 
   Therefore, multi-nozzle inkjet recording devices have been proposed for regulating the ejection velocity or ink droplet weight for each nozzle by making separate fine adjustments to the drive voltage waveform applied to the piezoelectric element or heating element of each nozzle. 
   For example, Japanese unexamined patent application publication No. HEI-9-11457 provides a multi-nozzle inkjet recording device having a plurality of drive waveform generators for generating desired drive voltage waveforms. In this multi-nozzle inkjet recording device, appropriate drive voltage waveforms are selected for each nozzle to achieve a desired ink droplet weight or ejection velocity, and the selected drive voltage waveform is applied to the nozzle from the corresponding drive waveform generator. 
   Further, Japanese unexamined patent application publication No. HEI-4-316851 provides a multi-nozzle inkjet recording device having a single drive waveform generator capable of generating a plurality of drive voltage waveforms. In this multi-nozzle inkjet recording device, since the same drive voltage waveform is applied to all nozzles simultaneously, it is not possible to eject ink simultaneously from all nozzles while applying individual drive voltage waveforms to each nozzle. Therefore, a time-division method is used to apply an appropriate drive voltage waveform sequentially to one nozzle at a time, obtaining the desired ink droplet weight or ejection velocity. 
   However, in the conventional multi-nozzle inkjet recording device described above, including a combination of Japanese unexamined patent application publication No. HEI-9-11457 and No. HEI-4-316851, it is not possible to perform calibration for both ejection velocity and ink droplet weight simultaneously Variations in the weight can increase when variations in velocity are suppressed, while variations in the velocity can increase when variations in weight are suppressed. 
   SUMMARY OF THE INVENTION 
   In view of the above-described drawbacks, it is an objective of the present invention to provide a multi-nozzle inkjet recording device capable of recording high-quality images by selectively emphasizing either precision in droplet ejection velocity or precision in ink droplet weight. 
   In order to attain the above and other objects, the present invention provides an inkjet recording device. The inkjet recording device includes a nozzle module, a switching unit, a waveform generating unit, an image recognizing unit and a pulse width modulating unit. 
   The nozzle module has a plurality of nozzles for ejecting ink droplets and a plurality of piezoelectric elements. Each piezoelectric element includes a common electrode and an individual electrode. The piezoelectric element is deformed when a potential difference is generated between the common electrode and the individual electrode. The nozzles are provided in one-to-one correspondence with the piezoelectric elements. Each nozzle ejects the ink droplet in accordance with deformation of the corresponding piezoelectric element. 
   The switching unit includes one terminal connected to the individual electrode and another terminal grounded. The switching unit is capable of opening and closing in response to a switch pulse. The opening duration of the switch unit is variable depending on the switch pulse. The waveform generating unit applies a drive voltage to the common electrodes of all the nozzles commonly. 
   The image recognizing unit determines an ejection condition of the ink droplet ejected from the nozzle while referring to ejection data indicating a type of each pixel to be recorded, and generates switch pulse width data that includes the ejection data and the ejection condition. The pulse width modulating unit generates the switch pulse based on the switch pulse width data. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects, features and advantages of the invention will become more apparent from reading the following description of the preferred embodiments taken in connection with the accompanying drawings in which: 
       FIG. 1  is a schematic diagram showing an overall ink ejection system according to a first embodiment of the present invention; 
       FIG. 2  is a cross-sectional view of an inkjet head module employed in the inkjet recording device according to a first embodiment; 
       FIG. 3  is a block diagram showing an inkjet drive circuit according to a first embodiment; 
       FIG. 4  is a block diagram showing an image recognizing device according to a first embodiment; 
       FIG. 5  is an explanatory diagram showing the order in which ejection data is transferred; 
       FIG. 6  is a schematic diagram showing switch pulse width data stored in a memory unit of the image recognizing device according to a first embodiment; 
       FIG. 7  is a explanation diagram showing a method of setting of the switch pulse width data; 
       FIG. 8  is a block diagram showing a pulse width modulating device according to a first embodiment; 
       FIG. 9  is a block diagram showing a waveform generator according to a first embodiment; 
       FIG. 10  is a timing chart showing the timing of operations performed in the inkjet drive circuit; 
       FIG. 11(   a ) is graphs showing an example of ink droplet velocity and weight characteristics in response to a nozzle ejection voltage; 
       FIG. 11(   b ) is graphs showing another example of ink droplet velocity and weight characteristics in response to a nozzle ejection voltage; 
       FIG. 11(   c ) is graphs showing another example of ink droplet velocity and weight characteristics in response to a nozzle ejection voltage; 
       FIG. 12  is an explanatory diagram showing the arrangement of inkjet head modules according to a second embodiment of the present invention; 
       FIG. 13  is a block diagram showing an inkjet head drive circuit according to the second embodiment; 
       FIG. 14  is a block diagram showing a switch pulse width data rearranging device according to the second embodiment; and 
       FIG. 15  is a block diagram showing a pulse width modulator according to a variation of the preferred embodiments. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   An inkjet-recording device according to a first embodiment of the present invention will be described while referring to  FIGS. 1 through 11 . 
     FIG. 1  shows the overall structure of an ink ejection system  1  equipped with an inkjet-recording device  10  according to the first embodiment. The ink ejection system  1  has a general structure similar to a common inkjet-recording system. As shown in  FIG. 1 , the ink ejection system  1  includes the inkjet-recording device  10  and a controller  20  such as a personal computer. 
   The inkjet-recording device  10  includes an inkjet head module (hereinafter referred to as a “head module”)  103 , a paper conveying device  105 , an inkjet head drive circuit (hereinafter abbreviated to “drive circuit”)  102 , and an ink tank  104 . A plurality (256 in the preferred embodiment) of nozzles  300  is arranged in a row in the head module  103 . The paper conveying device  105  conveys a recording paper  106  in a paper conveying direction A (indicated by the arrow A in the drawing) orthogonal to the row of nozzles  300  while outputting paper position detection signals ENC that indicate paper positions, to the controller  20 . The drive circuit  102  actuates the head modules  103  while transmitting a common drive voltage VCOM for all nozzles  300  and individual drive voltages VNOZ for each nozzle  300  in order to form an image on the recording paper  106 . The ink tank  104  supplies ink to the head modules  103  via a pipe. 
   The controller  20  outputs a latch enable signal LE, a data clock pulse CLK, and ejection data DAT to the drive circuit  102 . The latch enable signal LE is transmitted in synchronization with the paper position detection signal ENC in order to instruct start of forming of each line that configures a part of an image and is parallel to the row of nozzles  300 . The latch enable signal LE according to the preferred embodiment is a short pulse signal of 10 KHz. 
   The ejection data DAT is serial data with respect to ejection from each of the nozzles  300  arranged in order of 1 th  to 256 th  nozzle  300 . The ejection data DAT is “1” or “0”, where “1” represents ejection and “0” represents no ejection. The ejection data DAT is transmitted in synchronization with the data clock pulse CLK. The controller  20  begins transmitting the data clock pulse CLK and the ejection data DAT at the same instant of the transmitting of the latch enable signal LE. In the preferred embodiment, the data clock pulse CLK has a frequency of 5 MHz. Accordingly, 51.2 μs are required to transmit the 256 ejection data elements DAT for all of the nozzles  300 . 
   When the latch enable signal LE is generated, 256 bits of ejection data DAT, that has one-to-one correspondence with 256 of the nozzles  300  for the first line (line  1 ) of an image being recorded, is transferred. After one line worth of data has been transferred, 256 bits of data for the next line is transferred when the latch enable signal LE is generated again. The ejection data DAT for subsequent lines are transferred in the same way. 
   The head module  103  will be described with reference to  FIG. 2 .  FIG. 2  shows a part of the head module  103  corresponding to one nozzle  300 . The part of the head module  103  includes the nozzle  300 , an orifice plate  312 , a pressure chamber plate  311 , a restrictor plate  310 , a vibration plate  303 , a piezoelectric element fixing substrate  306  and a support plate  313 . The nozzle  300  includes a nozzle hole  301  (orifice) formed by the orifice plate  312 , a pressure chamber  302  formed by the pressure chamber plate  311 , and a restrictor  307  formed by the restrictor plate  310 . A common ink supply channel  308  for supplying ink to the pressure chamber  302  is formed in the nozzle module  103 . The restrictor  307  is in communication with the common ink supply channel  308  and pressure chamber  302  to control the amount of ink flow to the pressure chamber  302 . 
   Each nozzle  300  also includes a piezoelectric element  304 . One part of the piezoelectric element  304  is fixed to the piezoelectric element fixing substrate  306  and another part of the piezoelectric element  304  is linked to the vibration plate  303  by an elastic material  309 , such as a silicon adhesive. The piezoelectric element  304  includes a pair of signal input terminals  305   a  and  305   b . The piezoelectric element  304  expands and contracts when a voltage difference is generated between the signal input terminals  305   a  and  305   b , and remains in its original shape when a voltage is not applied. The support plate  313  reinforces the vibration plate  303 . 
   For example, the vibration plate  303 , restrictor plate  310 , pressure chamber plate  311 , and support plate  313  are made from stainless steel while the orifice plate  312  is constructed from a nickel material. The piezoelectric element fixing substrate  306  is formed of an insulating material, such as a ceramic or polyimide. 
   With this construction, ink supplied from the ink tank  104  ( FIG. 1 ) flows downward to each of the restrictors  307  via the common ink supply path  308  and is supplied into the pressure chambers  302  and nozzle holes  301 . When a voltage difference is generated between the signal input terminals  305   a  and  305   b , the piezoelectric element  304  deforms and a portion of the ink in the pressure chamber  302  is ejected through the nozzle hole  301 . 
   Next, the drive circuit  102  will be described with reference to  FIG. 3 . The drive circuit  102  includes an image recognizing device  201 , a shift register  203 , a latch  204 , a pulse width modulator  205 , a waveform generator  208 , and  256  switches  207 . The switches  207  have a one-to-one correspondence with the piezoelectric elements  304  (nozzles  300 ). 
   The image recognizing device  201  converts 1 bit ejection data DAT for each nozzle to 8 bit switch pulse width data  202  for modifying each nozzle&#39;s variation. The switch pulse width data  202  are stored in the shift register  203  sequentially in synchronization with the data clock pulse CLK. When all of the switch pulse width data  202  for the 256 nozzles  300  have been accumulated in the shift register  203  and the latch enable signal LE is generated, the latch  204  latches all of the switch pulse width data  202  accumulated in the shift register  203  simultaneously in synchronization with the latch enable signal LE. Then, the switch pulse width data  202  latched by the latch  204  is input into the pulse width modulator  205 . The pulse width modulator  205  converts the switch pulse width data  202  to a switch pulse  206 , and the switch pulse width data  202  is outputted to the corresponding signal input  207   a  of the switch  207 . 
   The upper side of each switch  207  is connected to the signal input terminal  305   b  of the corresponding nozzle  300 , while the lower side is grounded. If a “1” is inputted into the signal input  207   a , that is, if the switch pulse  206  is a “1”, the switch  207  closes. If a “0” is inputted into the signal input  207   a , that is, if the switch pulse  206  is a “0”, the switch  207  is opened. Thus, the individual drive voltages VNOZ1-VNOZ256 are applied to the signal input terminals  305   b  of each nozzle  300 . This will be described in greater detail below. 
   The waveform generator  208  generates a common drive voltage VCOM in synchronization with the latch enable signal LE. The common drive voltage VCOM is applied to the signal input terminals  305   a  of all the nozzles  300  commonly. 
   Next, the image recognizing device  201  will be described with reference to  FIG. 4 . The image recognizing device  201  includes a binary counter  401 , a memory unit  403 , FIFO memory units  405  and  407 , and flip flops  404   a - 404   f.    
   The binary counter  401  generates nozzle addresses  402  while counting the data clock pulse CLK. The first nozzle address  402  is “0” that indicates the first nozzle  300 , and the last nozzle address  402  is “255” that indicates the 256 th  nozzle  300 . The binary counter  401  is cleared by the latch enable signal LE. The nozzle addresses  402  are outputted to the memory unit  403 . Each nozzle address  402  corresponds to the ejection data DAT inputted into the memory unit  403  at same time. 
   The ejection data DAT inputted into the image recognizing device  201  is inputted into the memory unit  403  as the ejection data D 33  in synchronization with the data clock pulse CLK. The ejection data DAT is also inputted into the flip flop  404   a  and the FIFO memory unit  405  in synchronization with the data clock pulse CLK. 
   The ejection data DAT inputted into the flip flop  404   a  is inputted to the memory unit  403  as the ejection data D 32  in synchronization with the next data clock pulse CLK due to the storage function of the flip flop  404   a . The ejection data DAT inputted into the flip flop  404   a  is also inputted to flip flop  404   d . The ejection data DAT inputted into the flip flop  404   d  is also inputted to the memory unit  403  as the ejection data D 31  in synchronization with the further next data clock pulse CLK. 
   The FIFO memory unit  405  can store 8 bit worth of the ejection data DAT and has an internal address counter that is reset to 0 by the latch enable signal LE. The FIFO memory  405  does not output the ejection data DAT inputted until 8 bit worth of the ejection data DAT corresponding to one line has been stored. When the ejection data DAT corresponding to one line has been stored in the FIFO memory unit  405 , the FIFO memory unit  405  outputs ejection data DAT- 1  in synchronization with the data clock pulse CLK in order stored. Since the FIFO memory unit  405  outputs data inputted before 8 bit, the ejection data DAT- 1  corresponds to the previous line. 
   The ejection data DAT- 1  is inputted to the memory unit  403  as the ejection data D 23 , D 22  and D 21  in the same manner of D 33 , D 32  and D 31 . The ejection data DAT- 1  is also inputted into the FIFO memory unit  407 . The FIFO memory  407  outputs the ejection data DAT- 2  to the memory unit  403  as D 13 , D 12  and D 11  in the same manner. 
   The ejection data D 11 -D 33  obtained with this configuration indicates a region that is formed of a 3-by-3 (3×3) block of pixels in a recorded image as shown in  FIG. 5 . For example, D 11  is the first nozzle in the first line, D 12  is the second nozzle in the first line, D 13  is third nozzle in the first line, D 21  is first nozzle in the second line, D 22  is the second nozzle in the second line, D 23  is the third nozzle in the second line, D 31  is the first nozzle in the third line, D 32  is the second nozzle in the third line, and D 33  is the third nozzle in the third line. 
   The ejection data D 11 -D 33  are inputted all at once into the memory unit  403 . The memory unit  403  generates switch pulse width data  202  for each nozzle  300  corresponding to the ejection data D 22  that is a center of the region R. The memory unit  403  has stored switch pulse width table Tp for changing a flight condition, such as the quantity, of the ink droplet ejected from the nozzle  300  corresponding to the ejection data D 22  in question. The switch pulse width table Tp has switch pulse width data  202  with respect to the ejection data D 22  based on the condition of the ejection data D 11 -D 33  for all the nozzles. The switch pulse width data Tp has been obtained from experiments. 
   The memory unit  403  judges the condition of the ejection data D 22  based on the ejection data D 11 -D 21  and D 23 -D 33 . Meanwhile, the memory unit  403  judges that the state of the ejection data D 22  is which of (a) all of the ejection data D 11 -D 33  are black dots (“1”), (b) the ejection data D 22  is a black dot (“1”) though at least one of the ejection data D 11 -D 21  and D 23 -D 33  is a white dot (“0”), or (c) the ejection data D 22  is a white dot (“0”) without reference to D 11 -D 21  and D 23 -D 33 . Accordingly, it becomes that the memory unit  403  has stored switch pulse width table Tp that has the switch pulse width data  202  for each nozzle for each of (a), (b), (c) described above. 
     FIG. 6  shows the switch pulse width table Tp. In the preferred embodiment, the switch pulse width data  202  for each nozzle  300  is set to “Tp 1 -w” through “Tp 256 -w” in the case of (a). The switch pulse width data  202  for each nozzle  300  is set to “Tp 1 -v” through “tp 256 -v” in the case of (b). The switch pulse width data  202  for each nozzle  300  is set to “0” in the case of (c). 
     FIG. 7  shows a method of setting of the switch pulse width data  202 . In  FIG. 7 , the interval from LE N to LE N+1 is defined as line n, and the interval from LE N+1 to the LE N+2 (not shown) is defined as line (n+1). In  FIG. 7 , just the ejection data DAT for the first nozzle (nozzle address  402 =0) through the ninth nozzle (nozzle address  402 =8) in the line n are described for simplicity. In the present example, the ejection data DAT currently being transferred from the controller  20  is 001111100 . . . . Therefore, the ejection data D 33  is also 001111100. The ejection data D 32  is 000111110 . . . , since the ejection data D 32  is one dot behind of the ejection data D 33  due to the flip flop  404   a  ( FIG. 4 ). The ejection data D 31  is 000011111, since the ejection data is two dots behind of the ejection data D 33 . 
   The ejection data elements D 23 , D 22 , and D 21  in the current transfer are identical with the ejection data DAT- 1  transferred from the controller  20  one line earlier due to the FIFO memory  405  ( FIG. 4 ), though the ejection data DAT- 1  is the same as the ejection data DAT in the current transfer in the preferred embodiment. The ejection data elements D 13 , D 12 , and D 11  are identical with the ejection data DAT- 2  transferred two lines earlier due to the FIFO memory  405  and the FIFO memory  407 , though the ejection data DAT- 2  is the same as the ejection data DAT in the current transfer in the preferred embodiment. We will assume that all ejection data transferred three lines earlier or before are 0. 
   The first through third nozzles (nozzle addresses  402 =0-2) of the switch pulse width data  202  are “0” referring to the switch pulse width table Tp in  FIG. 6 , since the ejection data D 22  is “0. The fourth nozzle (nozzle address  402 =3) is “Tp 4 -v” and the eighth nozzle (nozzle address  402 =7) is “Tp 8 -v”, since the ejection data D 22  is “1” though at least one of the ejection data D 11 -D 21  and D 23 -D 33  is “0”. The fifth through seventh nozzles (nozzle addresses  402 =4-6) are “Tp 5 -w,” “Tp 6 -w,” and “Tp 7 -w”. The ninth nozzle (nozzle address  402 =8) and beyond are “0”. Note that this switch pulse width data  202  actually controls ejection for the next line (n+1), since this switch pulse width data  202  is latched in synchronous with the next latch enable signal LE N+1. 
   Next, the pulse width modulator  205  will be described with reference to  FIG. 8 . The pulse width modulator  205  includes 256 magnitude comparators  701  and a binary counter  702 . The magnitude comparators  701  have a one-to-one correspondence with the nozzles  300 . The switch pulse width data  202  outputted from the latch  204  (see  FIG. 2 ) is inputted into an input A of the corresponding magnitude comparator  701 . When the latch enable signal LE is inputted to the binary counter  702 , the binary counter  702  begins to count a high-frequency clock pulse HR-CLK generated by a crystal oscillator  90  from 0 to 255, and simultaneously outputs a signal  703  to inputs B of all the magnitude comparators  701 . The magnitude comparators  701  compare the magnitudes of the inputs A and B and generate a switch pulse  206 . The switch pulse  206  is “1” when A&gt;B while the switch pulse  206  is “0” when A≦B. 
   Next, the configuration of the waveform generator  208  will be described with reference to  FIG. 9 . The waveform generator  208  includes a binary counter  801 , a waveform memory unit  802 , a digital/analog (D/A) converter  805  that is well known in the art, an op-amp circuit  806 , and an amplifier  807 . When the latch enable signal LE is inputted to the binary counter  801 , the binary counter  801  begins to count a high-frequency clock pulse HF-CLK 2  generated by a crystal oscillator  60 , and simultaneously outputs the count to the waveform memory unit  802 . The waveform memory unit  802  outputs output waveform data  804  previously stored therein to the D/A converter  805 . The D/A converter  805  converts the output waveform data  804  to an analog signal. The analog signal is amplified by the op-amp circuit  806  and amplifier  807  and is applied to the signal input terminal  305   a  of each nozzle  300  as the common drive-voltage VCOM. 
   Next, operations of the pulse width modulator  205  will be described for the fifth nozzle  300  (nozzle address  402 =4) referring to  FIG. 10 .  FIG. 10  shows a timing chart for operations of the pulse width modulator  205 . In  FIG. 10 , the interval from LE N to LE N+1 is defined as line n, and the interval from LE N+1 to LE N+2 (not shown) is defined as line (n+1). In the preferred embodiment, when the latch enable signal LN is inputted into the binary counter  702 , the binary counter  702  begins to count from 0 to 255 and simultaneously outputs the signal  703  to the input B of the magnitude comparator  701 . Tp 5 -v as the switch pulse width data  202  for line n is inputted into the input A of the magnitude comparator  701  in synchronization with the latch enable signal LE N, and Tp 5 -w is inputted for line (n+1) in synchronization with the latch enable signal LE N+1. Note that Tp 5 -v is not shown at the switch pulse width data  202  in  FIG. 10  since the Tp 5 -v outputted in line n is generated at line n−1. 
   The magnitude comparator  701  is comparing the magnitudes of inputs A and B each time the binary comparator  702  is incremented. The magnitude comparator  701  outputs “1” to the signal input  207   a  as switch pulse  206  when the input A is larger than the input B, while outputting “0” to the signal input  207   a  as switch pulse  206  when the input A is smaller than the input B. The switch  207  closes when “1” is inputted into the signal input  207   a , while the switch  207  is opened when “0” is inputted into the signal input  207   a.    
   The waveform generator  208  also outputs the common drive voltage VCOM shown in  FIG. 10  in synchronization with the latch enable LE N. The piezoelectric element  304  can be viewed as a capacitor. When the switch  207  closes (t 1 ), the potential difference between the signal input terminal  305   a  and the signal input terminal  305   b  is the common drive voltage VCOM itself since the signal input terminal  305   b  is grounded. On the other hand, when the switch  207  is opened (t 2 ), the potential difference between the signal input terminal  305   a  and the signal input terminal  305   b  since current cannot flow. As a result, the potential VNOZ5 is applied to the signal input terminal  305   b . Consequently, the difference potential V (VCOM-VNOZ5) between the common drive voltage VCOM and the potential VNOZ5 is applied to the piezoelectric element  304 . Hence, the pulse width modulator  205  outputs the switch pulse  206  to the signal input  207   a  of the corresponding switch  207 . Meanwhile, a voltage V corresponding to the duration of the switch pulse  206  is applied to the piezoelectric element  304 , since the switch  207  closes only when “1” is inputted to the signal input  207   a.    
   The waveform of the drive voltage V is a trapezoidal wave well known in the art. When the voltage V drops, the pressure chamber  302  expands, drawing the meniscus inside the nozzle hole  301 . When the voltage V rises (the voltage difference is called as an ejection voltage Vf), the pressure chamber  302  contracts, causing the meniscus to move outward. Thus, an ink droplet is ejected. The ejection velocity v and droplet weight w of the ink droplet ejected from the nozzle  300  varies according to the ejection voltage Vf. 
     FIG. 11(   a ) shows the ejection velocity v and droplet weight w when the ejection voltage Vf for ejecting ink droplets is fixed at a constant value for all of the nozzles  300  (1 st  through 256 th  nozzles). As can be seen from the graph, the ejection velocity v increases for nozzles  300  near both ends, while in contrast the droplet weight w decreases. 
     FIG. 11(   b ) shows the ejection velocity v and droplet weight w when the ejection voltage Vf has been adjusted to achieve a constant ejection velocity v for all ink droplets. The ejection voltage in this case is called the ejection voltage Vf-v. Since both the ejection velocity v and droplet weight w generally increase when increasing the ejection voltage Vf, the droplet weight w varies more among nozzles in this case than in the case of  FIG. 11(   a ). 
     FIG. 11(   c ) shows the ejection velocity v and droplet weight w when the ejection voltage Vf has been adjusted to achieve a constant droplet weight w ejected from all the nozzles. The ejection voltage in this case is called the ejection voltage Vf-w. Since both the ejection velocity v and droplet weight w generally increase when increasing the ejection voltage Vf as described above, the ejection velocity v varies more among nozzles in this case than in the case of  FIG. 11(   a ). 
   The “Tp 1 -v” through “Tp 256 -v” and the “Tp 1 -w” through “Tp 256 -w” stored in the memory unit  403  corresponds to the ejection voltage Vf-v and ejection voltage Vf-w for each nozzle. 
   In the preferred embodiment, it is possible to switch the priority for precision in droplet weight and precision in ejection velocity automatically for each pixel. Meanwhile, which of the precision in droplet weight or the precision in ejection velocity is determined based on the ejection data D 11 -D 33  referring to the switch pulse width table Tp. 
   Since the ink droplet weight for each nozzle is fixed when printing a solid image (case (a)), it is possible to prevent streaks and other printing problems in the paper conveying direction A caused by irregularities in density. As a result, the quality of images can be improved. The quality of halftone images can similarly be improved by recording all dots in a halftone image at the same weight. 
   Since the ink droplet velocity for each nozzle is fixed when printing text or diagrams, such as graphs and tables (case (b)), it is possible to record high-quality images at a high rate of speed with no variation in the ejection positions. 
   Therefore, it is possible to achieve high quality printing of composite images. 
   Next, an ink ejection system according to a second embodiment of the present invention will be described with reference to  FIGS. 12-14 . Here, only a description of points different from the ink ejection system of the first embodiment will be given, while a description of common points will be omitted. 
   In the ink ejection system according to the second embodiment, as shown in  FIG. 12 , the head modules  103  are slanted in the clockwise direction from the paper conveying direction A, that is, the y-direction in  FIG. 12  (the longitudinal dimension of the paper surface) by an angle θ (where tan θ=¼). This method of mounting the head modules  103  in a slanted orientation is a common technique to achieve high-density image recording when a pitch Pn between nozzles  300  in the nozzle rows is too large. If the recording pitch in the paper conveying direction A is Pp, then:
 
 Pp=Pn  sin θ
 
   Although exaggerated in  FIG. 12 , the head modules  103  of the preferred embodiment are arranged so that the recording pitch in the x- and y-directions achieves a ratio of 1:4. While it is possible to secure a wide recording width by arranging a plurality of head modules  103  in the x-direction, in the following description it will be assumed that there is only one head module  103 . 
   The ink ejection system according to the second embodiment includes a drive circuit  1102  in place of the drive circuit  102 , as shown in  FIG. 13 . The drive circuit  1102  is configured almost identically to the drive circuit  102 , but is also provided with a switch pulse width data switching device (hereinafter abbreviated to “switching device”)  1200  disposed between the latch  204  and pulse width modulator  205 . 
   If the switch pulse width data  1202  for all the nozzles  300  are inputted into the pulse width modular  205  simultaneously such as the first embodiment when the head modules  103  is slanted, a line is also formed slanted since the ejection data DAT is data with respect to the X-direction in  FIG. 12 . Therefore, the switching device  1200  adjusts the timing that each nozzle  300  ejects an ink droplet. 
     FIG. 14  shows a detailed configuration of the switching device  1200 . The switching device  1200  includes 255 FIFO memory units  2001 - 2255 , each having a capacity of four lines worth (four LEs worth) of data. 
   The latch  204  outputs a 256×8-bit latch output  1202  (switch pulse width data  202 ) for the 1 st  through 256 th  nozzles to the switching device  1200 . Of this data, only 8 bits for the first nozzle (Tp 1 ) is transferred to the pulse width modulator  205 , while the remainder (255×8 bits) is inputted into the FIFO memory unit  2001 . The FIFO memory unit  2001  outputs the remainder of the latch output  1202  for four lines earlier (255×8 bits) as output  2001 ′. Of this output data, only 8 bits for the 2 nd  nozzle (Tp 2 ) is transferred to the pulse width modulator  205 . 
   The remainder of the output  2001 ′ (254×8 bits) is inputted into the FIFO memory unit  2002 . Hence, the FIFO memory unit  2002  outputs the remainder of the latch output  1202  for eight lines earlier as output  2002 ′. Of this output data, only 8 bits for the 3 rd  nozzle (Tp 3 ) is transferred to the pulse width modulator  205 . 
   After repeatedly performing this process, the final remainder (1×8 bits) is inputted into the FIFO memory unit  2255 . Hence, the FIEO memory unit  2255  outputs the remainder of the latch output  1202  for 4×255 lines earlier (1×8 bits), which output is transferred to the pulse width modulator  205  as 8 bits for the 256 th  nozzle (Tp 256 ). 
   Thus, each ink droplet ejected from each nozzle  300  is ejected while delayed so that the ink droplets ejected from all the nozzle  300  form a line in the X-direction. Accordingly, in the preferred embodiment, when the head module  103  is disposed at a slant in order to record at a desired resolution, the switching device  1200  can rearrange the switch pulse width data  202  in order to achieve the same effects obtained in the first embodiment described above. 
   While the invention has been described in detail with reference to specific embodiments thereof, it would be apparent to those skilled in the art that many modifications and variations may be made therein without departing from the spirit of the invention, the scope of which is defined by the attached claims. 
   For example, although the switch pulse width data  202  in the preferred embodiments described above is 8 bits in size, the switch pulse width data  202  may be set to any number of bits. When the switch pulse width data  202  is less than 8 bits, memory units  1301  may be disposed in direct connection to the inputs A of the magnitude comparators  701  to convert the switch pulse width data  20  from n bits to 8 bits, as shown in  FIG. 15 . Meanwhile, the switch pulse width data  202  is converted to a more detailed switch pulse width data  202 . 
   Further, while only one head module  103  was described in the first and second embodiments, a plurality of head modules  103  may be provided. Though the switch pulse width data  202  is generated based 3×3 blocks (D 11 -D 33 ) in the preferred embodiment, more blocks may be referred to generate the switch pulse width data  202 .