Patent Publication Number: US-7905564-B2

Title: Ink jet printer capable of forming high definition images

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a divisional of U.S. application Ser. No. 11/730,484, filed Apr. 2, 2007, which is a divisional of U.S. application Ser. No. 09/057,502, filed Apr. 9, 1998, which claims the benefit of application No. 9-092252 filed in Japan on Apr. 10, 1997, the contents of which are hereby incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates generally to ink jet printers, and more particularly, to an ink jet printer capable of smoothing images. 
     There are known some ink jet printers using a piezoelectric element (PZT) for a print head. In such a print head, pulse voltage corresponding to image data is applied to the piezoelectric element, and the piezoelectric element deforms in response to the application of the pulse voltage, which pressurizes ink within a prescribed container (ink channel) and permits ink droplets to be ejected from a nozzle provided at the ink channel toward a recording sheet. An image based on the image data is formed on the recording sheet by the ejected ink droplets. 
     In the ink jet printer, the amount of liquid to form ink droplets to be ejected is adjusted by causing degree of distortion at the piezoelectric element by changing the amplitude of the pulse voltage applied to the piezoelectric element. Thus adjusting the amount of liquid to form ink droplets, a plurality of dot sizes are available for ink to stick to a recording sheet. Among the plurality of dot sizes, larger dot sizes are used to represent a dark part of an image, and smaller sizes are used to represent a light part of the image. 
     Meanwhile, in the field of ink jet printers, a smoothing process of virtually improving the resolution of an image and improving a jaggy part of the image at the time of reproducing the image from image data is performed. In the smoothing process, smaller size dots as described above are used. 
     Referring to  FIGS. 29 and 30 , the smoothing process will be described.  FIG. 29  is a diagram for use in illustration of printing of an image by a normal ink jet printer. 
     An image printed by the ink jet printer is virtually divided into segments, dots  251  to  254  having a plurality of sizes as described above are printed for printing an image having a density. In the image, the dot center-to-center distance, the distance between the center of a certain dot and the center of an adjacent dot in the four sides is fixed regardless of the size of the dots. In the conventional ink jet printer thus printing images performs the following smoothing process. 
       FIG. 30  is a diagram for use in illustration of a smoothing process by a conventional ink jet printer. 
     In the conventional ink jet printer, an image segmented into a lattice is subjected to a smoothing process, in which smaller size smoothing dots  256  are printed around a normal size dot  255 . 
     If, however, smaller size dots are printed in the smoothing process as described above, the dot center-to-center distance may appear to be separated in some printed images. In such an image, the effect of smoothing process deteriorates, in other words, high definition image is not available to the user. 
     SUMMARY 
     It is therefore one object of the invention to provide an ink jet printer capable of recording high definition images. 
     Another object of the invention is to provide a method of controlling printing in an ink jet printer, according to which high definition images can be recorded. 
     The above-described objects of the invention are achieved by an ink jet printer including the following elements. More specifically, the ink jet printer according to the present invention ejects a plurality of kinds of ink droplets having different sizes depending upon data to be printed, and forms an image on a prescribed recording medium using dots of sizes corresponding to the sizes of the ink droplets. The ink jet printer includes a smoother for smoothing an image using dots smaller than the dots forming the image, and a controller for controlling the smoother to print the smaller dots at positions close to the image forming dots a smaller pitch than the dot pitch of the image. 
     The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view showing a general structure of an ink jet printer according to a first embodiment of the invention; 
         FIG. 2  is a plan view of a plane having nozzles in an ink jet head; 
         FIG. 3  is a cross sectional view taken along line III-III in  FIG. 2 ; 
         FIG. 4  is a cross sectional view taken along line IV-IV in  FIG. 3 ; 
         FIG. 5  is a perspective view for use in illustration of the structure of the periphery of a carriage; 
         FIG. 6  is a block diagram showing a general configuration of the control unit of the ink jet printer; 
         FIG. 7  is a block diagram for use in illustration of the flow of processes performed to image data; 
         FIG. 8  is a chart showing the waveform of pulse voltage applied from an ejection driving portion for driving a piezoelectric element; 
         FIG. 9  is a graph showing the speed of ejection of ink droplets ejected by applying the pulse voltage shown in  FIG. 8  to a piezoelectric element; 
         FIG. 10  is a graph showing the volume of ink droplets ejected by application of the pulse voltage shown in  FIG. 8  to a piezoelectric element; 
         FIG. 11  is a graph showing the size of dots formed by ink droplets ejected by application of the pulse voltage shown in  FIG. 8  to a piezoelectric element and sticking to a recording medium; 
         FIG. 12  is a diagram showing examples of dots printed by application of the pulse voltage shown in  FIG. 8 ; 
         FIGS. 13 and 14  are diagrams for use in illustration of a smoothing process by the ink jet printer according to the first embodiment of the invention; 
         FIG. 15  is a chart for use in illustration of the timing of printing smoothing dots; 
         FIG. 16  is a chart for use in illustration of application of pulse voltage to a piezoelectric element for printing smoothing dots by the ink jet printer according to the first embodiment of the invention; 
         FIG. 17  is a flow chart for use in illustration of the procedure of processes by a smoothing determination portion executed by a CPU  101 ; 
         FIG. 18  is a chart showing the waveform of pulse voltage applied to drive a piezoelectric element by an ink jet printer according to a second embodiment of the invention; 
         FIG. 19  is a graph showing the speed of ejection of ink droplets by applying the pulse voltage shown in  FIG. 18  to a piezoelectric element; 
         FIG. 20  is a graph showing the volume of ink droplets ejected by application of the pulse voltage shown in  FIG. 18  to a piezoelectric element; 
         FIG. 21  is a chart showing the size of dots sticking to a recording medium formed by ink droplets ejected by application of the pulse voltage shown in  FIG. 18  to a piezoelectric element; 
         FIG. 22  is a chart for use in illustration of printing of dots shifted in position because of difference in the speed of ejection; 
         FIG. 23  is a chart for use in illustration of the timing of printing smoothing dots; 
         FIG. 24  is a chart for use in illustration of application of pulse voltage to a piezoelectric element for printing smoothing dots by the ink jet printer according to the second embodiment of the invention; 
         FIG. 25  is a chart showing the waveform of pulse voltage applied to drive a piezoelectric element in an ink jet printer according to a third embodiment of the invention; 
         FIG. 26  is a graph showing the speed of ejection of ink droplets ejected by applying the pulse voltage shown in  FIG. 25  to a piezoelectric element; 
         FIG. 27  is a graph showing the volume of ink droplets ejected by applying the pulse voltage shown in  FIG. 25  to a piezoelectric element; 
         FIG. 28  is a chart showing the size of dots sticking to a recording medium formed by ink droplets ejected by application of the pulse voltage shown in  FIG. 25  to a piezoelectric element; 
         FIG. 29  is a diagram for use in illustration of printing of images by a normal ink jet printer; and 
         FIG. 30  is a graph for use in illustration of a smoothing process by a conventional ink jet printer. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An ink jet printer according to a first embodiment of the invention will be now described in conjunction with the accompanying drawings. 
     Referring to  FIG. 1 , an ink jet printer  1  includes an ink jet head  3 , an ink jet type print head for printing images onto a recording sheet  2 , a recording medium such as a paper sheet or OHP sheet, a carriage  4  for carrying ink jet head  3 , swinging shafts  5  and  6  for moving carriage  4  back and forth parallel to the recording plane of recording sheet  2 , a driving motor  7  for driving carriage  4  to move back and forth along swinging shafts  5  and  6 , a timing belt  9  for converting the rotation of driving motor  7  into the reciprocating movement of carriage  4 , and an idle pulley  8 . 
     Ink jet printer  1  includes a platen  10  also serving as a guide plate for guiding recording sheet  2  along a transport path, a paper pressing plate  11  for preventing recording sheet  2  between platen  10  and itself from being lifted, a discharge roller  12  for discharging recording sheet  2 , a spur roller  13 , a regaining system  14  for cleaning the nozzle surface of ink jet head  3  which ejects ink, thereby returning an ink ejection fault to a good state, and a paper feeding knob for manually transporting recording sheet  2 . 
     Recording sheet  2  is fed into a recording unit in which ink jet head  3  and platen  10  oppose each other manually or by a paper feeding device such as a cut sheet feeder which is not shown. At the time, the amount of rotation of a paper feeding roller which is not shown is controlled, so that the transfer into the recording unit is controlled. 
     A piezoelectric element (PZT) is used in ink jet head  3  as a source of generating energy for ejection of ink. The piezoelectric element is supplied with voltage and distorts. The distortion changes the volume of a channel filled with ink. The change in the volume of the channel allows ink to be ejected from a nozzle provided at the channel, so that recording to recording sheet  2  is performed. Recording sheet  2  is set at a prescribed position and fed in its lengthwise direction. 
     Carriage  4  scans recording sheet in the width direction corresponding to the main scanning direction by the function of driving motor  7 , idle pulley  8  and timing belt  9 . Ink jet head  3  attached to carriage  4  records images for one line. Each time data for one line is recorded, recording sheet  2  is fed in the longitudinal direction for sub scanning, and data in the next line is recorded. 
     Images are thus recorded on recording sheet  2 , which is then passed through the recording unit, and discharged by discharge roller  12  provided on the downstream side in the transporting direction and spur roller  13  in contact with roller  12  under prescribed pressure. 
     Referring to  FIGS. 2 to 5 , in jet head  3  and its peripheral structure of ink jet head  3  will be now described. 
       FIGS. 2 to 4  are views for use in illustration of ink jet head  3 . 
       FIG. 2  is a plan view showing a plane having nozzles of ink jet head  3 ,  FIG. 3  is a cross sectional view taken along line III-III in  FIG. 2 , and  FIG. 4  is a cross sectional view taken along line IV-IV in  FIG. 3 . 
     Ink jet head  3  is formed by a nozzle plate  301 , a partitioning wall  302 , a vibrating plate  303 , and a substrate  304  which are integrally placed upon each other. 
     Nozzle plate  301  is formed of a metal or ceramics and has a nozzle  307  and an ink repellent layer on its surface  318 . Partitioning wall  302  is formed of a thin film and is fixed between nozzle plate  301  and vibrating plate  303 . 
     There are provided between nozzle plate  301  and partitioning wall  302  a plurality of ink channels  306  for storing ink, and an ink inlet  309  coupling each ink channel  306  to an ink supply chamber  308 . Ink supply chamber  308  is connected to an ink tank which is not shown, and ink  305  in ink supply chamber  308  is supplied to ink channels  306 . 
     Vibrating plate  303  includes a plurality of piezoelectric elements  313  corresponding to ink channels  306 . Vibrating plate  303  is fixed to substrate  304  having an interconnection portion  317  by an insulating adhesive, and then separate grooves  315  and  316  are formed by dicing to segment vibrating plate  303 . By the segmentation, a piezoelectric element  313  corresponding to each ink channel  308 , a piezoelectric pillar portion  314  positioned between adjacent piezoelectric elements  313 , and a peripheral wall  310  surrounding these elements are separated from each other. 
     Interconnection portion  317  on substrate  304  has a common electrode side interconnection portion  311  connected to ground and connected commonly to piezoelectric elements  313  in ink jet head  3 , and an individual electrode side interconnection portion  312  individually connected to each piezoelectric element  313  in ink jet head  3 . Common electrode side interconnection portion  311  on substrate  304  is connected to a common electrode in piezoelectric elements  313 , and individual electrode side interconnection portion  312  is connected to an individual electrode in piezoelectric element  313 . 
     The operation of thus structured ink jet head  3  is controlled by a control unit in ink jet printer  1 . A printing signal at a prescribed voltage is applied from the ejection driving portion  106  of the control unit (see in  FIG. 6 ) across the region between the common electrode and each individual electrode provided in piezoelectric element  313 , and piezoelectric element deforms in the direction pressing partitioning wall  302 . The deformation of piezoelectric element  313  is transmitted to partitioning wall  302 , which pressurizes ink  305  in ink channel  306 , and ink droplets are ejected through nozzle  307  toward recording sheet  2  (see  FIG. 1 ). 
       FIG. 5  is a perspective view for use in illustration of the structure of the periphery of carriage  4 . The periphery of carriage  4  includes an ink cartridge  403  for storing ink and having a ventilation hole  404 , a casing  401  for storing ink cartridge  403 , a casing lid  405 , an ink supply pin  403  for allowing ink cartridge  403  to be detached and supplying ink to ink jet head  3 , a clutch  406  for fixing casing lid  405  at casing  401  when casing lid  405  is closed, an energizing clutch stopper  407 , and a plate spring  408  for pressing ink cartridge  403  in the opposite direction to the direction of storing ink cartridge  403  (the direction denoted by arrow D 3 ) and retaining cartridge  403  together with casing lid  406 . As carriage  4  moves in the direction denoted by D 1 , a recording sheet is scanned in the main scanning direction, and ink droplets are ejected in the direction denoted by D 2 . 
     The ink in ink cartridge  403  includes, as solvent, 80.9% of water, 11.0% of polyhydric alcohol/diethylene glycol, and 2.5% of a viscosity enhancer/polyethylene glycol #400, as a color agent, 4.6% of dye/Bayer BK-SP, and as additive, 0.8% of a surface active agent/olefin E1010, and 0.2% of a pH controlling agent/NaHCO3. Ink  305  having this composition exhibits a surface tension of 36 (dyn/cm) at 25° C., and a viscosity of 2.0 (cp), and a super fine sheet manufactured by the Epson Corporation is used for recording paper (recording sheet  2 ). 
     Now, the control unit of ink jet printer  1  will be described.  FIG. 6  is a block diagram for use in illustration of the configuration of the control unit in ink jet printer  1 . 
     A CPU (Central Processing Unit)  101  in the control unit of ink jet printer  1  is connected to a storage portion  102  including a ROM (Read Only Memory) and a RAM (Random Access Memory), an interface portion  103  connected to a host  20  such as a computer or a word processing machine to exchange data, a sensor detection portion  104 , a display operation portion  105 , an ejection driving portion  106 , a carriage motor driving portion  107 , and a sheet feeding motor driving portion  108 . 
     Control programs to control ink jet printer  1  are stored in the ROM in storage portion  102 , and the ROM includes a character generator. The RAM in storage portion  102  includes a receiving buffer for temporarily storing data transferred from host  20  and a print buffer for developing the received data into data to be actually printed and temporarily storing the data. 
     Sensor detection portion  104  includes sensors necessary for detecting the position of the carriage, the temperature and the presence/absence of a recording sheet, and display operation portion  105  includes a display lamp, and various operation switches. 
     CPU  101  controls the print head, carriage motor and sheet feeding motor through ejection driving portion  106 , carriage motor driving portion  107 , and sheet feeding motor driving portion  108 , respectively based on various input data detection signals and records images on a recording sheet. 
       FIG. 7  is a block diagram for use in illustration of the flow of processes performed to image data. These processes are executed by CPU  101  in  FIG. 6 . 
     Image data input from host  20  in  FIG. 6  is analyzed by a command analyze portion  111 . If the input image data is character data, the data is read out from a CG memory  112 , and bit map data is developed in the print buffer by a developing modifying portion  113 . If the input image data is picture data, the image data is developed in the print buffer by an image data developing processing portion  114 . 
     After the processes, it is determined by a smoothing setting determination portion  115  if a smoothing process is to be performed to the data in the print buffer. If the smoothing process has not been set, the control proceeds to succeeding process  117  without performing a smoothing process to the data in the print buffer, while if a smoothing process has been set, the data in the print buffer is subjected to the smoothing process at smoothing portion  116 , and then the control proceeds to succeeding process  117 . In succeeding process  117 , the image data after the smoothing process is converted into data for driving a piezoelectric element, and ejection driving portion  106  (see  FIG. 6 ) is controlled based on the data to drive the piezoelectric element. 
     From ejection driving portion  106  in ink jet printer  1  as described above, pulse voltage having a waveform as shown in  FIG. 8  is applied to piezoelectric element  313  (see  FIGS. 2 to 4 ). 
       FIG. 8  is a chart showing the waveform of pulse voltage applied from ejection driving portion  106  to drive the piezoelectric element. Herein, the tone of image to be printed has five tone levels, waveforms start to be applied at the same time point in a graph in which the ordinate represents voltage and the abscissa represents time from the start of application of voltage, and waveforms A 1 , A 2 , . . . , and A 5  have ascending pulse amplitudes in this order. 
     The results of measuring the speed of ejection of ink droplets, the volume of droplets, and the size of dots sticking to a recording sheet in response to application of pulse voltage having waveforms A 1  to A 5  to a piezoelectric element are given in  FIGS. 9 to 11 . The speed of ejection, the droplet volume, and the dot sticking size are average values produced by printing 100 dots, and the ink and the recording sheets used were the same as those described in conjunction with  FIG. 5 . 
       FIG. 9  is a graph showing the speed of ejection of ink droplets ejected in response to application of the pulse voltage shown in  FIG. 8  to the piezoelectric element,  FIG. 10  is a graph showing the volume of ink droplets ejected in response to application of the pulse voltage shown in  FIG. 8  to a piezoelectric element, and  FIG. 11  is a graph showing the size of dots sticking to a recording sheet formed by ink droplets ejected in response to application of the pulse voltage shown in  FIG. 8  to the piezoelectric element. In these figures, the abscissa represents the pulse amplitude of the pulse voltage shown in  FIG. 8 , and the ordinate represents the speed of ejection of ink droplets, the volume of droplets and the size of sticking dots in response to these pulse amplitudes. 
     As shown in  FIGS. 10 and 11 , as the pulse amplitude increases in the pulse voltage having waveforms A 1  to A 5  in  FIG. 8 , the volume of corresponding ink droplets and the size of sticking dots both increase. As shown in  FIG. 9 , the speeds of ejection of ink droplets corresponding to waveforms A 1  to A 5  are almost fixed at 5 m/s regardless of the size of the ink droplets. 
       FIG. 12  is a graph showing examples of dots printed in response to application of the pulse voltage shown in  FIG. 8 . 
     Dots  201 ,  202  and  203  having different sizes correspond to waveforms A 1 , A 3  and A 5 , respectively in  FIG. 8  and printed while maintaining the center-to-center distance in the lattice formed by virtual segments on an image at an almost fixed level for scanning at a fixed speed. The center-to-center distance among the different size dots  201 ,  202  and  203  is maintained at an almost fixed level, because the speed of ejection of corresponding ink droplets, the speed of scanning of the carriage and the driving frequency of the piezoelectric element are maintained at a fixed level. 
       FIG. 13  is a first diagram for use in illustration of a smoothing process by the ink jet printer according to the first embodiment of the invention. For dot  204  printed in a normal timing (based on a fixed driving frequency of the piezoelectric element), a smoothing dot  205  may be printed closer to dot  206  (at a shorter center-to-center distance) to be smoothed than dot  204  printed in the normal timing by setting earlier the timing of application of voltage to the piezoelectric element. Herein, arrow D 4  denotes the direction of scanning. 
       FIG. 14  is a second diagram for use in illustration of the smoothing process by ink jet printer  1  according to the first embodiment of the invention. 
     Dots  221  to  226  are smoothed using smoothing dots A 211  to A 213  and smoothing dots B 214  to B 216 . During the smoothing, smoothing dots A 211  to A 213  are printed in a timing delayed from that of normal dots relative to scanning direction D 4 , while smoothing dots B 214  to B 216  are printed in a timing earlier than that of normal dots relative to scanning direction D 4 . In practice, these timings may be produced as follows. 
       FIG. 15  is a chart for use in illustration of the timing of printing smoothing dots. Herein, the size of a dot  232  to be smoothed is 100 μm, the piezoelectric element is driven at a pulse amplitude of 15V when smoothing dot  231  is printed, the size of the smoothing dot is 60 μm, the dot is printed at 250 dpi (at a dot interval of 100 μm) onto a recording sheet, the scanning speed of the carriage is 250 mm/s, and the distance between the nozzle surface of the ink jet head and the recording sheet is 1 mm. In addition, regardless of the size of ink droplets, the speed of ejection of the ink droplets is fixed at 5 m/s. 
     The center-to-center distance of normal dot is 100 μm, but the center-to-center distance between dot  232  to be smoothed and smoothing dot  231  is set to 80 μm under the above-described condition (at the time, dot  232  and smoothing dot  231  are in contact). The timing of applying pulse voltage to the piezoelectric element which is changed for shortening the center-to-center distance is produced as follows. 
     If a normal dot is printed without smoothing, the center-to-center distance between dots is 1.00 μm, the scanning speed of the carriage is 250 mm/s, and therefore time until the next dot is printed after a certain dot is printed is produced by the following expression:
 
0.1/250=4×10 −4  [s]=0.4 [ms]
 
     The driving frequency of the piezoelectric element is produced as 2.5 kHz from the inverse of the time. When a smoothing is performed, the center-to-center distance between dots is 80 μm, and time since a certain dot is printed until a smoothing dot therefor is printed is produced by the following expression:
 
0.08/250=3.2×10 −4  [s]=0.32 [ms]
 
     From the above two expressions, the following expression is produced:
 
0.4−0.32=0.08 [ms]
 
     By printing a dot in a timing earlier (or delayed) than normal, a smoothing dot having a shorter center-to-center distance to a dot to be smoothed may be printed. 
       FIG. 16  is a chart for use in illustration of application of pulse voltage to the piezoelectric element for printing a smoothing dot by the ink jet printer according to the first embodiment of the invention. 
     Waveform  501  is for printing a normal dot  204  in  FIG. 13 , pulse voltage applied to the piezoelectric element for printing smoothing dots A 211  to A 213  in  FIG. 14  have a waveform  502 , and pulse voltage applied to the piezoelectric element for printing smoothing dots B 214  to B 216  in  FIG. 14  have a waveform  503 . 
     In order to select these waveforms  501  to  503 , the following control (which corresponds to the process at smoothing determination portion  115  in  FIG. 1 ) is executed by CPU  101  (see  FIG. 6 ). 
       FIG. 17  is a flow chart for use in illustration of the procedure of processes by smoothing determination portion  115  executed by CPU  101 . 
     In S 1 , a variable dn (the number attached sequentially from an end of a line) for specifying each dot in line n, (a set of linearly arranged dots) in the n-th line forming an image to be printed is set to 1, in other words dn=1. In S 2 , the data of dots specified by dn is referred to. 
     In S 3  and S 4 , based on the data of dots corresponding to dn referred to in S 2 , it is determined if a smoothing to any of adjacent dots is necessary. If it is determined that a smoothing process is necessary to a dot adjacent at the right (YES in S 3 ), a variable Tdn indicating whether a smoothing process is necessary is set to 1 in S 5 , in other words Tdn=1, while if it is determined that a smoothing process is necessary to a dot adjacent at the left (NO in S 3 , and YES in S 4 ), Tdn is set to 2, in other words, Tdn=2 in S 6 . If it is determined that a smoothing process is not necessary (NO in S 3  and S 4 ), Tdn is set to 0, in other words Tdn=0 in S 7 . 
     When Tdn is substituted by any of 0, 1 and 2, the value of Tdn is stored for each line in a printer buffer A in S 8 . It is determined in S 9  if the n-th line has been finished and if data for 1 line has been stored in the buffer (YES in S 9 ), the routine is completed, while if data for 1 line has not been stored in the buffer (NO in S 9 ), dn is added with 1 in S 10  and the processes from S 2  are repeated. 
     The waveform of pulse voltage applied to the piezoelectric element (the timing of applying the pulse voltage) is selected for each dot in each line forming the image to be printed, and stored in printer buffer A for each line. The size of dots to be printed is 60 μm for smoothing dots, and determined based on the result of a tone process such as dither process when dots other than smoothing dots are printed, and data representing the size of dots is stored in a printer buffer B. 
     The data representing the time of applying pulse voltage stored in printer buffer A and the data representing the size of dots stored in printer buffer B are used for printing. 
     As described above, during smoothing a dot to be printed, the timing of printing is changed, a smaller size dot is printed close to a dot to be smoothed, and therefore the center-to-center distance between the dot to be smoothed and the smoothing dot will not appear to vary as experienced by the conventional device, so that high definition images may be recorded. 
     Ink jet printers according to second and third embodiments of the invention will now be described. The ink jet printer according to the second and third embodiments of the invention will be described particularly from viewpoints of difference from the ink jet printer according to the first embodiment of the invention by referring to the drawings, the general structures of the ink jet printer, ink jet head, control unit and the other elements including the procedure of control at the control unit are similar to the ink jet printer according to the first embodiment of the invention. 
       FIG. 18  is a chart showing the waveform of pulse voltage applied to drive a piezoelectric element in an ink jet printer according to the second embodiment. Herein, the tone of an image to be printed has eight tone levels.  FIG. 18  corresponds to  FIG. 8  for the ink jet printer according to the first embodiment. Herein, waveforms B 1 , B 2 , . . . and B 8  have ascending pulse amplitudes in this order. 
     The results of measuring the speed of ejection of ink droplets, the volume of droplets and the size of dots sticking to a recording sheet by applying pulse voltage having waveforms B 1  to B 8  are given in  FIGS. 19 to 21 .  FIGS. 19 to 21  correspond to  FIGS. 9 to 11  for the ink jet printer according to the first embodiment, the measurement condition, and the method of displaying data are the same as those for the ink jet printer according to the first embodiment. 
     As shown in  FIGS. 20 and 21 , as the pulse amplitude increases among the pulse voltage having waveforms B 1  to B 8  in  FIG. 18 , the volume of corresponding droplets, and the size of corresponding dots both increase. Also as shown in  FIG. 19 , the speed of ejection of ink droplets corresponding to waveforms B 1  to B 8  are almost fixed for those corresponding to waveforms B 4  to B 8 , while the ejection speed increases as the pulse amplitude increases for those corresponding to waveforms B 1  to B 3  having smaller pulse amplitudes and smaller ink droplet sizes. If the ejection speed thus differs, the position of printing is shifted if the piezoelectric element is driven at a fixed driving frequency by a carriage having a fixed scanning speed. 
       FIG. 22  is a chart for use in illustration of printing of dots shifted in position because of difference in the ejection speed. 
     If a large size ink droplet, and a small size ink droplet in a different ejection speed from the large size ink droplet are ejected to the scanning direction D 4  of the carriage, a large size dot  251  and a small side dot  252  are printed on a recording sheet accordingly, but small size ink droplets take more time to reach the recording sheet than the large size ink droplets, the distance of movement of the carriage in scanning direction D 4  is larger. Thus, the center of the small size ink droplet is at a position shifted toward scanning direction D 4  from the center of the large size ink droplet in virtual segments in a lattice on the recording sheet. 
     Thus, if the speed of ejection of ink droplets is different depending on the size of ink droplets, two parameters, in other words the speed of ejection of ink droplets and the speed of scanning of the carriage should be taken into account, in order to change the position of printing a smoothing dot. 
       FIG. 23  is a chart for use in illustration of the timing of printing of a smoothing dot. A dot  261  is smoothed by a smoothing dot C 262  and a smoothing dot D 263 , the corresponding ink droplets of which have smaller size and smaller ejection speed. Dots  264  and  265  are dots printed in a normal timing (based on a fixed driving frequency of the piezoelectric element which is the same as that for printing dot  261 ). 
     During smoothing such dot  261 , smoothing dot C 262  is printed in a timing delayed from the timing of printing dot  264  in scanning direction D 4 , and smoothing dot D 263  is printed in a timing earlier than that of printing dot  265  in scanning direction D 4 . In practice, these timings may be produced as follows: 
     Herein, the size of dot  261  to be smoothed is 100 μm, the size of smoothing dots  262  and  263  is 40 μm, the dots are printed at 250 dpi (the dot distance is 100 μm) on a recording sheet, and the distance between the nozzle surface of the ink jet head and the recording sheet is 0.5 mm. The speed of ejection of ink droplets for printing smoothing dots  262  and  263  (dots  264  and  265 ) is 3 m/s. The scanning speed of the carriage is 250 mm/s the same as that of the ink jet printer according to the first embodiment, and the driving frequency of the piezoelectric element is 2.5 kHz. 
     The moving distance of an ink droplet corresponding to dot  261  in scanning direction D 4  until the ink droplet reaches a recording sheet from the nozzle surface of the ink jet head is given as follows:
 
250×(0.5/5000)=0.025 [mm]
 
     The moving distance of ink droplets corresponding to dots  264  and  265  in the scanning direction until the ink droplets reach a recording sheet from the nozzle surface of the ink jet head is given as follows:
 
250×(0.5/3000)≈0.042 [mm]
 
     As a result, it is understood that the center of dots  264  and  265  are printed shifted from the center of segments in a lattice by the following amount in scanning direction D 4 :
 
0.042−0.025=0.017 [mm]
 
     In order to print dot  262 , the dot must be moved in scanning direction D 4  more than dot  264  by the following amount:
 
30−17=13 [μm]
 
     As a result, the dot must be printed in a timing delayed by the following amount from the normal timing.
 
0.013/250=5.2×10 −5  [s]≈0.05 [ms]
 
     In order to print dot  263 , the dot must be moved in a direction opposite to scanning direction D 4  from dot  265  by the following amount:
 
17+30=47 [μm]
 
     Therefore, the dot must be printed in a timing earlier than the normal timing by the following amount:
 
0.047/250=1.9×10 −4  [s]≈0.19 [ms]
 
     By changing the printing timings as described above, a smoothing dot having a shorter center-to-center distance to a dot to be smoothed may be printed. 
       FIG. 24  is a chart for use in illustration of application of pulse voltage to the piezoelectric element to print smoothing dots by the ink jet printer according to the second embodiment. 
     Waveform  551  is for printing normal dots  264  and  265  in  FIG. 23 , pulse voltage applied to the piezoelectric element to print a smoothing dot C 262  in  FIG. 23  has a waveform  552 , and pulse voltage applied to the piezoelectric element to print a smoothing dot D 263  in  FIG. 23  has a waveform  553 . 
     Note that in the case of the ink jet printer according to the second embodiment, the speed of ejection changes depending upon the size of the smoothing dot, and therefore the timing of application of the pulse voltage to the piezoelectric element should be changed depending upon the speed of ejection as follows. A table of dot sizes and ejection speeds is provided in smoothing portion  116  (see  FIG. 7 ), and the timing of printing may be changed according to the table. 
     As in the foregoing, during smoothing a dot to be printed, the timing of printing is changed, and smaller size dots are printed close to the dot to be smoothed, so that the center-to-center distance between the dot to be smoothed and the smoothing dot will not appear to vary as experienced by the conventional device, and therefore high definition images may be recorded. 
       FIG. 25  is a chart showing the waveform of pulse voltage applied to drive a piezoelectric element by an ink jet printer according to a third embodiment of the invention. Herein, the tone of an image to be printed has five tone levels as is the case with the ink jet printer according to the first embodiment, and the effect of smoothing and an image to be printed by smoothing are similar to those shown in  FIGS. 13 and 14 . Waveforms  601  to  605  have ascending pulse amplitudes in this order, and the waveforms of pulse voltage corresponding to smoothing dots are waveforms  606  and  607 . 
     It is clear from experiments that the speed of ejection of ink droplets increases as voltage raised per unit time is larger. The speed of ejection of an ink droplet according to waveform  606  is set lower than the speed of ejection of an ink droplet according to waveform  601 , and the speed of ejection of an ink droplet according to waveform  607  is higher than the speed of ejection of an ink droplet according to waveform  601 . 
     Thus, the ejection speed is set lower than normal, smoothing dots  211  to  213  as shown in  FIG. 14  having their centers shifted in scanning direction D 4  relative to a dot to be printed (dot  204  in  FIG. 13 ) may be printed by applying pulse voltage having normal waveform  601  to the piezoelectric element. Also thus setting higher the ejection speed than normal, smoothing dots  214  to  216  as shown in  FIG. 14  having their centers shifted in a direction opposite to scanning direction D 4  relative to a dot to be printed by applying pulse voltage having normal waveform  601  as shown in  FIG. 25  to the piezoelectric element result. 
     The results of measuring the speed of ejection of ink droplets, the volume of the droplets, and the size of dots sticking to a recording sheet by applying pulse voltage having waveforms  601  to  605  to the piezoelectric element are given in  FIGS. 26 to 28 .  FIGS. 26 to 28  correspond to  FIGS. 9 to 11  for the ink jet printer according to the first embodiment of the invention, and the measuring condition, and the way of displaying data are the same as those for the ink jet printer according to the first embodiment. 
     As shown in  FIGS. 27 and 28 , as the pulse amplitude increases in pulse voltage having waveforms  601  to  605  shown in  FIG. 25 , the volume of corresponding ink droplets and the size of corresponding dots both increase. As shown in  FIG. 26 , the speeds of ejection of ink droplets corresponding to waveforms  601  to  605  are almost fixed at 5 m/s regardless of the size of ink droplets. 
     As in the foregoing, during smoothing a dot to be printed, the speed of ejection of corresponding ink droplets are changed, smaller size dots are printed close to the dot to be smoothed, so that the center-to-center distance between the dot to be smoothed and the smoothing dot will not appear to be separated as experienced by the conventional device, and high definition images can be recorded. 
     It is understood that, in the case without a smoothing process, if the speed of ejection of ink droplets changes depending upon the piezoelectric element, the two parameters, the speed of ejection of ink droplets and the scanning speed may be taken into account as is the case with the ink jet printer according to the second embodiment, and dots may be printed at appropriate positions. 
     In the foregoing, the embodiment is described with reference to a single integrated printer. However, the present invention is not limited to the foregoing but applicable to the ink jet printing device used as a recording portion of a copying machine, a facsimile and so on. 
     Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims.