Abstract:
An inkjet printing apparatus having a print head and a control system to control the discharge of variable size ink drops from the print head. The control system controls a print operation to divide the printing of an image into steps, wherein at least a first step includes the printing of dots having diameters within a specified range and at least a second step includes the printing of dots having diameters within another specified range.

Description:
BACKGROUND OF THE INVENTION 
     Conventionally, there are known inkjet printers having print heads provided with a piezoelectric element (PZT). For such print heads, a pulse voltage corresponding to image information is applied to the piezoelectric element, causing a predetermined distortion of the piezoelectric element. This distortion pressurizes ink inside an adjacent container, or ink channel, and an ink drop is discharged from the ink channel toward a print sheet, thus forming a printed ink dot. A plurality of printed ink dots yields an image on the print sheet. 
     A printer of this type also creates image gradations by discharging ink drops of different diameters. Different diameter ink drops are achieved by varying the amount of change (degree of distortion) of the piezoelectric elements. Specifically, by reducing the amount of change in the piezoelectric element, an ink drop of a smaller diameter is discharged, and by increasing the amount of change in the piezoelectric element, an ink drop of a larger diameter is discharged. 
     Interestingly, as the amount of change in the piezoelectric element increases, the speed of discharge of a corresponding ink drop from the print head increases. Therefore, ink drops of different diameters have different travel speeds. Conventionally, piezoelectric elements are driven in constant cycles, regardless of the size of the ink drops to be expelled. As an example of the issues created by this arrangement, where the discharge of ink drops of a large diameter follow the discharge of small diameter ink drops, the distance between the centers of printed dots having small diameters and the printed dots having large diameters is smaller than the distance which is ordinarily obtained. In this way, a shift occurs in the positions of the printed dots due to the difference in the travel speeds of the discharged ink drops. This shift causes a marked deterioration in image quality. 
     FIGS. 19 through 21 illustrate the deterioration in image quality due to a shift in dot position caused by differing ink drop travel speeds following discharge. 
     FIGS.  19 ( a ) and  19 ( b ) illustrate the deterioration in image quality due to a shift in dot position in a printer using the dither method. FIG.  19 ( a ) shows a standard dither method, while FIG.  19 ( b ) shows gradations using the dither method when a shift in dot position has occurred. The numbers in FIGS.  19 ( a ) and  19 ( b ) represent a gradation number. Arrow D 3  (FIG.  19 ( a )) represents the direction of scanning of a print head. 
     Assuming the travel speeds of a large diameter ink drop and a small diameter ink drop are equal, the dot pattern is printed as shown in FIG.  19 ( a ). However, as set forth above, the travel speed of a small diameter ink drop is less than that of a large diameter ink drop. Consequently, as shown in FIG.  19 ( b ), a small diameter ink drop experiences a shift in position in a direction opposite the direction of scanning D 3 . 
     Moreover, in actual printing, the different ink drop travel speeds may cause a large diameter ink dot and a small diameter ink dot to overlap in an image having, for example, a gradation level  5 , which causes said image to be undesirably lighter than an image having a gradation level  4 . 
     FIG. 20 further illustrates the deterioration of image quality due to dot deformation. Because the travel speed of a small diameter ink drop is less than that of a large diameter ink drop, part of a small diameter printed ink dot may merge with an adjacent large diameter printed ink dot. The result is a gourd-shaped dot. Understandably, if a number of such gourd-shaped dots are printed, the printed image will have poor granularity and appear rough. 
     FIG. 21 also illustrates the deterioration in image quality when a cyclical pattern is printed. Where lines of small diameter printed ink dots and lines of large diameter printed ink dots are alternately printed, the small diameter ink dots are formed closer to the large diameter dots than desired. This occurrence is obvious given that the travel speed of the small diameter ink drops is less than that of the large diameter ink drops. Consequently, the distances between lines will not be constant and the printed image will have cyclical noise. 
     The problem and examples described above arise from the differing travel speeds of ink drops having different sizes. At least one technique to equalize different travel speeds using an air flow is well known. 
     FIG. 22 illustrates both the structure and technique utilizing air flow to equalize ink drop travel so as to establish a constant travel speed. The area that expels an ink drop includes piezoelectric element  201 , ink channel  202 , nozzle  203 , and air flow path  204 . Piezoelectric element  201 , which is distorted via the application of a pulse voltage, causes the pressurization of ink inside ink channel  202 . The ink thus pressurized is discharged through nozzle  203  as ink drop  205 . The travel speed of ink drop  205  may be made constant by blowing air into air flow path  204 , located in front of nozzle  203 , in the manner shown by arrow  206 . 
     While this technique has shown an ability to equalize different ink drop travel speeds, printing devices which incorporate this system are complex, which leads to an increase in the cost of manufacture. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming the shortcomings of the present art. The present invention is drawn to an inkjet printing apparatus for forming an image on a printing medium. The inkjet printing apparatus includes a print head, to discharge ink drops to form an image in accordance with image data, and a controller. The controller, drives the print head in a first scanning direction to discharge first ink drops having a first travel speed and also drives the print head in a second scanning direction to discharge second ink drops having a second travel speed. 
     In another manner, the inkjet printing apparatus of the present invention includes a print head, for the purposes set forth above, and a controller to effect the print head to (i) discharge ink drops in a first scanning direction to form printed ink dots of a first diameter and (ii) discharge ink drops in a second scanning direction to form printed ink dots of at least a second diameter. 
     An object of the present invention is to provide an inkjet printing device that can maintain good image quality using a simple construction. 
     Another object of the present invention is to prevent printing irregularities associated with the discharging of ink drops having different traveling speeds. 
     Another object of at least one embodiment of the present invention is to utilize the natural scanning motion of a print head to avoid printing irregularities associated with the discharge of ink drops which form printed ink dots of variable diameters. 
     Other objects and advantages of the present invention will be apparent to those of ordinary skill in the art having reference to the following specification together with the drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Referring now to the drawings in which like reference numerals and letters indicate corresponding elements throughout the several views, if applicable: 
     FIG. 1 is a perspective view of an inkjet printer according to an embodiment of the present invention; 
     FIG. 2 is a plan view illustrating a print head of the present invention; 
     FIG. 3 is a sectional view taken along line III—III of the print head of FIG. 2; 
     FIG. 4 is a sectional view taken along line IV—IV of the print head of FIG. 3; 
     FIG. 5 is a block diagram of a control system of the inkjet printer of the present invention; 
     FIG. 6 illustrates a first embodiment of carriage control for printing in accordance with the present invention; 
     FIG. 7 is a flow chart of a control sequence for the control system of the present invention; 
     FIG. 8 illustrates a group of pulse voltage waveforms A 1 -A 6  for application to the piezoelectric elements of the print head of the present invention; 
     FIG. 9 illustrates ink drop travel speeds discharged from a print head of the present invention in accordance with the pulse voltages of FIG. 8; 
     FIG. 10 illustrates ink drop volumes discharged from a print head of the present invention in accordance with the pulse voltages of FIG. 8; 
     FIG. 11 illustrates printed ink dot diameters from ink drops discharged from a print head of the present invention in accordance with the pulse voltages of FIG. 8; 
     FIGS.  12 ( a )- 12 ( c ) illustrate an example of printed ink dots in accordance with the first and second embodiments of the present invention; 
     FIGS.  13 ( a )- 13 ( d ) illustrate a second embodiment, a third embodiment, and a fourth embodiment, respectively, of carriage control for printing in accordance with the present invention; 
     FIG. 14 illustrates a group of pulse voltage waveforms B 1 -B 8  for application to the piezoelectric elements of the print head of the present invention in accordance with the third embodiment; 
     FIG. 15 illustrates ink drop travel speeds discharged from a print head of the present invention in accordance with the pulse voltages of FIG. 14; 
     FIG. 16 illustrates ink drop volumes discharged from a print head of the present invention in accordance with the pulse voltages of FIG. 14; 
     FIG. 17 illustrates printed ink dot diameters from ink drops discharged from a print head of the present invention in accordance with the pulse voltages of FIG. 14; 
     FIGS.  18 ( a )- 18 ( c ) illustrate an example of printed ink dots in accordance with at least the third embodiment of the present invention; 
     FIGS.  19 ( a ) and  19 ( b ) illustrate image quality deterioration caused by a shift in the positions of printed ink dots when using a dither method; 
     FIG. 20 illustrates image quality deterioration caused by printed ink dot deformation; 
     FIG. 21 illustrates image quality deterioration when printing a cyclical pattern using a conventional inkjet print system; and 
     FIG. 22 illustrates conventional technology using a forced air flow to make the travel speed of discharged ink drops equal. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An inkjet printer according to an embodiment of the present invention will be described below with reference to the drawings. 
     FIG. 1 is a perspective view schematically showing the construction of an inkjet printer  1  according to an embodiment of the present invention. The inkjet printer  1  includes an inkjet type print head  3 ; a carriage  4  for holding the print head  3 ; shafts  5  and  6  for reciprocating the carriage  4  in parallel with a printing surface of a print medium  2 ; a driving motor  7  for reciprocating the carriage  4  along the shafts  5  and  6 ; a timing belt  9  for transforming the rotation of the driving motor  7  into a reciprocating motion of the carriage  4 ; and an idling pulley  8 . The inkjet printer  1  accommodates a print medium  2 , or a print sheet, wherein a print sheet  2  may be a paper sheet (for example, Superfine™ paper, Epson Corporation), a thin, plastic sheet or film, or the like. 
     The carriage  4  is reciprocated by a combination of the driving motor  7 , the idling pulley  8 , and the timing belt  9  in the directions D 1  and D 2 , wherein the print head  3  mounted thereto successively prints images one line at a time. Every time the printing of one line is completed, the print sheet  2  is fed in its lengthwise direction to allow printing of a next line and to generate an image on the print sheet  2 . 
     The inkjet printer  1  further includes a platen  10  which concurrently serves as a guide plate for guiding the print sheet  2  along a transfer path; a sheet pressing plate  11  for pressing the print sheet  2  against the platen  10  to prevent lifting; a discharging roller  12  for discharging the print sheet  2 ; a spur roller  13 ; a recovering system  14  for recovering a defective ink discharge of the print head  3 ; and a paper feeding knob  15  for manually feeding the print sheet  2 . 
     A print sheet  2  is fed either manually or by a paper feeding unit (not shown), such as a cut sheet feeder, into a printing section where the print head  3  and the platen face each other. In this stage, the amount of rotation of a paper feeding roller (not shown) controls the feeding of the print sheet  2  into the printing section. 
     The print head  3  of the inkjet printer  1  and its periphery will be described next with reference to FIGS. 2 through 5. FIGS. 2,  3 , and  4  illustrate the print head  3  of the present invention. Specifically, FIG. 2 is a plan view of the print head  3 , FIG. 3 is a section view taken along the line III—III of the print head  3  of FIG. 2, and FIG. 4 is a section view taken along the line IV—IV of the print head  3  of FIG.  3 . 
     Referring to FIG. 3, the print head  3  is constructed of a nozzle plate  301 , a membrane  302 , a piezoelectric member  303 , and a base plate  304  in an integrally stacked configuration. 
     The nozzle plate  301  is constructed of metal, synthetic resin, ceramic, or alike material. A surface  308  of nozzle plate  301 , which faces membrane  302 , is finely finished by electroforming, photolithography or the like, so that a plurality of recesses are formed. These recessions establish a plurality of ink channels  306  for storing ink  305 ; an ink supplying chamber  308  that contains resupply ink  305 ; and ink inlets  309  that connect ink channels  306  to ink supplying chamber  308 . This finished surface  308  is further provided with an ink repellent layer, for example, a Teflon® coating (DuPont Corp., Wilmington, Del.). 
     The ink channels  306  are elongated in a lateral direction with respect to print head  3  and are arranged in parallel in a direction perpendicular to such lateral direction. The ink supplying chamber  308 , positioned tone side of the ink channels  306 , is in fluid communication with an ink tank (not shown) and operates to supply ink  305  from the ink tank to ink channels  306 . Extending from an outer surface of print head  3  to ink channels  306 , nozzles  307  are positioned at an end of the ink channels  306  opposite the ink supply chamber  308 . In at least one embodiment, nozzles  307  are convergently tapered, where the ink channel-side diameter is wider than the exit diameter. 
     In a preferred embodiment, ink  305  is a composition including 77.0% water, 6.5% diethylene glycol (DEG), 6.5% triethylene glycol monobutyl ether (TGB), 4.5% thickener (PEG #400) as a solvent, 4.5% pigment (Bayer&#39;s BK-SP) as a coloring agent, and 0.8% surfactant (Olfine E1010) and 0.2% pH adjusting agent (NaHCO 3 ) as additives. 
     Returning to the construction of the print head  3 , membrane  302  is formed of a thin film material and is fixed between the nozzle plate  301  and the piezoelectric member  303 . As a specific portion of the piezoelectric member  303  corresponds to each ink channel  306  and such portions are made to deform for the purpose of discharging an ink drop from such ink channels  306  (as will be discussed in greater detail below), the membrane  302  does not prevent the deformation of the piezoelectric member  303  portions but rather yields so as to transmit such deformation to ink channels  306 . 
     The piezoelectric member  303  of the present invention is formed from a piezoelectric element (PZT), wherein the piezoelectric member  303  serves as an energy source for discharging ink  305  from the print head  3 . Generally, a voltage is applied to a specific portion of the piezoelectric member  303 , resulting in a distortion of such portion. Each piezoelectric member  303  portion corresponds to an ink channel  306 . Accordingly, the distortion of a piezoelectric member  303  portion effects a change in volume in its corresponding ink channel  306  containing ink  305 . By such change in volume, ink  305  is discharged through a nozzle  307 . 
     The piezoelectric member  303  is fixed between the membrane  302  and the base plate  304 . A conductive adhesive is used to join at least the piezoelectric member  303  and the base plate  304 , wherein the piezoelectric member  303  is joined to the base plate  304  with respect to a wiring section  317 . Prior to the membrane  302  being fixed in place, the piezoelectric member  303  is cut longitudinally and laterally in a dicing process, producing a series of longitudinal grooves  315  and lateral grooves  316 . Consequently, the piezoelectric member  303  is separated into piezoelectric elements  313  corresponding to each ink channel  306 ; partition walls  314  positioned between adjacent piezoelectric members  313 ; and peripheral walls  310  which encloses these members. 
     On a surface of the base plate  306  which faces the piezoelectric member  303 , a wiring section  317  is provided having a common electrode section  311  and an individual electrode section  312 . The common electrode section  311  is electrically coupled to ground and each of the piezoelectric members  313 , and the individual electrode section  312  is electrically coupled to head expulsion drive unit (FIG. 5) and to each of the piezoelectric members  313 . 
     FIG. 5 is a block diagram of a control system of the inkjet printer  1  of the present invention. 
     Central processing unit (CPU)  101  of the control unit of inkjet printer  1  is connected to memory unit  102 , interface unit  103 , sensor detection unit  104 , display operation unit  105 , head expulsion drive unit  106 , carriage motor drive unit  107 , and sheet feeder motor drive unit  108 . CPU  101  controls print head  3 , carriage motor  7 , and the sheet feeder motor by means of head expulsion drive unit  106 , carriage motor drive unit  107 , and sheet feeder motor drive unit  108 , respectively, to effect the printing of an image on a print sheet  2 . 
     Memory unit  102  includes a ROM (read-only memory) and RAM (random access memory). The ROM of memory unit  102  houses control programs to control inkjet printer  1  and also includes a character generator. The RAM of memory unit  102  includes a receiving buffer (not shown), that temporarily stores data transmitted from host  20 , as well as a print buffer (not shown), that temporarily stores data that is to be actually printed and which is generated from the expansion of the received data. The RAM of memory unit  102  is also used as a work area when the control programs are executed. 
     Interface unit  103  is connected to host  20 , which is a computer, word processor or the like, such that data can be transmitted and received. Sensor detection unit  104  includes sensors necessary to detect the position of the carriage  4 , the temperature, the existence of a printing sheet and so on. Display operation unit  105  includes a display lamp and various operational switches. 
     Carriage control and printing performed by the inkjet printer  1  described above are explained below in the forms of first through fourth embodiments. Carriage control and printing are explained for the first embodiment in reference to FIGS.  6  through  12 ( a )- 12 ( c ), and carriage control and printing for the second through fourth embodiments are explained with reference to FIGS.  13  through  18 ( a )- 18 ( c ). 
     First, carriage control and printing for the first embodiment will be explained with reference to FIGS. 6 through 12. 
     FIG. 6 illustrates the carriage control for ink dot printing for a first embodiment of the present invention. More particularly, the inkjet printer  1  of this embodiment prints ink dots, other than those ink dots having a smallest diameter, while carriage  4  moves in a forward direction (direction D 1  in FIG.  1 ). Inkjet printer  1  prints ink dots having the smallest diameter only while carriage  4  moves in an opposite direction (direction D 2  in FIG.  1 ). Such carriage control and printing are achieved by means of CPU  101  performing the control shown in FIG.  7 . 
     FIG. 7 is a flowchart to explain the control sequence of CPU  101 . More specifically, the flowchart of FIG. 7 serves to execute carriage control in accordance with FIG.  6 . 
     Referring to both FIG.  5  and FIG. 7, when executing a print operation, first, CPU  101  passes image data for one print line from host  20  to the RAM of memory unit  102  (via interface unit  103 ), as shown in step S 1 . Then, in step S 2 , as a part of processing the image data, as input in step S 1 , into data to be actually printed, CPU  101  calculates the gradation level that corresponds to each piece of image data. 
     In step S 3 , CPU  101  allocates to a first buffer (not shown) of the RAM of memory unit  102  items of print data having a gradation level using ink dot diameters other than a smallest diameter. The print data allocated and stored in the first buffer is for printing in a forward (D 1 ; FIG. 6) direction. CPU  101  further allocates print data having a gradation level using ink dots of the smallest diameter to a second buffer (not shown) of the RAM of memory unit  102 . The print data allocated and stored in the second buffer is for printing in a backward (D 2 ; FIG. 6) direction. 
     In step S 4 , CPU  101  effects forward scanning of carriage  4  while discharging ink drops in accordance with the print data stored in the first buffer. In step S 5 , CPU  101  effects backward scanning of carriage  4  while discharging ink drops in accordance with the print data stored in the second buffer. 
     It is then determined in step S 6  whether or not a page has been printed on print sheet  2 . If an image for one page has not been printed (NO in step S 6 ), CPU  101  returns to step S 1 . If an image for one page has been printed (YES in step S 6 ), this routine comes to an end, whereupon CPU  101  returns to the main routine to perform other control operations. 
     The image data stored in the first and second buffers define particular pulse voltages, as will be discussed in greater detail below, which are applied to print head piezoelectric elements to cause the discharge of ink drops. 
     FIG. 8 shows a group of pulse voltage waveforms output from head expulsion drive unit  106  (FIG. 5) to effect the discharge of ink. The pulse voltages are shown together on a coordinate system in which the vertical axis represents the voltage and the horizontal axis represents the time that has elapsed since the commencement of voltage application. The voltages are numbered as waveforms A 1 -A 6 , based on different pulse amplitudes. As a reference, the waveform A 1 , having the smallest pulse amplitude, produces the smallest diameter ink dot of waveforms A 1 -A 6 . Accordingly, waveforms A 2 -A 6  correspond to ink dots other than the smallest diameter ink dot. Waveforms A 1 -A 6  are capable of producing six gradations. 
     FIGS. 9-11 present information regarding ink drop travel speed, ink drop volume, and printed ink dot diameter for ink drops formed in accordance with the present invention and discharged in accordance with waveforms A 1 -A 6 . The travel speeds, drop volumes, and dot diameters shown in these figures represent average values obtained from the printing of 100 dots. 
     FIG. 9 shows the travel speed of ink drops discharged in accordance with the pulse voltages of FIG.  8 . FIG. 10 shows the volume of ink drops discharged in accordance with pulse voltages of FIG.  8 . FIG. 11 shows the printed diameter of ink dots formed from ink drops discharged in accordance with the pulse voltages in FIG.  8 . In these figures, the horizontal axis represents the pulse amplitude of the waveforms shown in FIG. 8, while the vertical axes of FIGS. 9-11 respectively represent the travel speed, drop volume, and printed dot diameter for each different level of pulse amplitude. 
     Referring to FIG. 9, the travel speed of an ink drop stays essentially constant at 5 m/s for waveforms A 2  through A 6 . In contrast, the travel speed of an ink drop for waveform A 1  is 4 m/s, which is approximately 20% less than the travel speed of an ink drop for waveforms A 2  through A 6 . Moreover, as shown in FIGS. 10 and 11, as the pulse amplitude (pulse voltage intensity) of waveforms A 1 -A 6  increases, the discharged ink drop volume and printed ink dot diameter increase. In particular reference to FIG. 11, the approximately 60 μm diameter of printed ink dots formed in accordance with waveform A 1  are necessary for creating gradations such as halftones. 
     It is assumed that printing takes place at 250 dpi for discharged ink drops having travel speeds consistent with the above example. The distance between the carriage  4  and the print sheet  2  is approximately 1 mm. The scan speed of the carriage is 250 mm/s. The piezoelectric element drive frequency during printing is 2.5 kHz. 
     As provided above, carriage  4  is moved forward (direction D 1 ) to print ink dots produced by waveforms A 2 -A 6 . Carriage  4  is moved backward to print ink dots produced by waveform A 1 . When this is done, the piezoelectric elements are driven faster, such increase being as much as the difference in travel speed relative to dots having diameters other than the smallest diameter. In other words, since the travel speed for “larger” ink dots (i.e., those produced by waveforms A 2 -A 6 ) is 5 m/s and the travel speed for the smallest diameter dots is 4 m/s, the difference in arrival at the printing sheet is {fraction (1/4000)}-{fraction (1/5000)}=0.05 [ms]. Therefore, the piezoelectric elements are driven such that the smallest diameter ink dots are expelled 0.05 ms sooner than those ink dots produced by waveforms A 2 -A 6 . 
     During printing, the absolute position of carriage  4  within the scan path is detected by means of an encoder (not shown) positioned on carriage  4 . This system prevents a shift in position between the printed ink dots that are printed when the carriage  4  moves forward (direction D 1 ) and the printed ink dots that are printed when the carriage  4  moves backward (direction D 2 ). Incidentally, when performing printing of the smallest diameter dots corresponding to waveform A 1 , a 2.5 kHz piezoelectric element drive frequency may be maintained. 
     Using such carriage control and printing in this manner, shifts in position for small diameter ink dots during printing may be prevented and good image quality may be maintained while utilizing a simple print head construction. 
     FIGS.  12 ( a )- 12 ( c ) illustrate ink dots printed on a print sheet  2  using the first embodiment. FIG.  12 ( a ) shows printed ink dots that are printed during the forward and backward scanning of carriage  4 . FIG.  12 ( b ) shows printed ink dots that are printed during the forward scanning of carriage  4 . FIG.  12 ( c ) shows printed ink dots that are printed during the return scanning of carriage  4 . Functionally, ink dots having a specified diameter or greater are printed as print head  3  moves in a forward direction (FIG.  12 ( b )) and, during a return pass by print head  3 , ink dots having the smallest diameter are printed (FIG.  12 ( c )), thus producing a combined image as shown in FIG.  12 ( a ). 
     Carriage control and printing for the second through fourth embodiments will now be explained with reference to FIGS. 13 through 18. 
     FIGS.  13 ( a ),  13 ( b ) and  13 ( c ) correspond to the second, third, and fourth embodiments, respectively, of the present invention. In the second through fourth embodiments, the CPU  101  (FIG. 5) executes control in a manner similar to that set forth in the flowchart of FIG.  7 . Differences in control are set forth in detail below. 
     In the second embodiment and in reference to FIG.  13 ( a ), a group of ink dots having a variety of possible diameters (such group excluding a smallest diameter) are printed while carriage  4  moves in a forward scanning direction (D 1 ). Following the carriage  4  returning to a “home” position through rearward movement (direction D 2 ), the smallest diameter ink dots are printed as the carriage  4  again moves in the forward scanning direction (D 1 ). At least for this example, the pulse voltages to drive the piezoelectric elements are the same as those for the first embodiment. 
     In the third embodiment and in reference to FIG.  13 ( b ), a group of ink dots having larger diameters, i.e., a large-diameter region, are printed while carriage  4  moves in a forward direction (D 1 ). A group of ink dots having smaller diameters, i.e., a small-diameter region, are printed while carriage  4  moves in a rearward direction (D 2 ). The large-diameter regions and the small-diameter regions are discussed in greater detail below. 
     In the fourth embodiment and in reference to FIG.  13 ( c ), ink dots in the large-diameter region are printed while carriage  4  moves in a forward direction (D 1 ). Similar to the second embodiment, carriage  4  thereafter returns to a “home” position through rearward movement (direction D 2 ), wherein ink dots in the small-diameter region are printed while carriage  4  again moves in a forward direction (D 1 ). At least for this example, the pulse voltages to drive the piezoelectric elements are the same as those for the third embodiment. 
     The large-diameter and small-diameter regions, referred to in connection with the third and fourth embodiments, allow ink dots having different diameters to be divided into a group of ink dots that have relatively large diameters and a group of ink dots that have relatively small diameters, wherein, in a preferred embodiment, each group has multiple ink dot diameters possible. 
     Similar to that illustrated in FIG. 8, FIG. 14 shows a second group of pulse voltage waveforms for output from head expulsion drive unit  106  (FIG.  5 ). The waveforms of FIG. 14 are capable of producing eight gradations. The voltages are numbered as waveforms B 1 -B 8 , based on different pulse amplitudes. As a reference, the waveform B 1 , having the smallest pulse amplitude, produces the smallest diameter ink dot of waveforms B 1 -B 8 . Defining the regions of waveforms B 1 -B 8 , waveforms B 1  through B 3  belong to a small-diameter region, and waveforms B 4  through B 8  belong to a large-diameter region. 
     Measurements identical to the measurements that were obtained for the first embodiment, the results of which were shown in FIGS. 9-11, were also conducted for the third embodiment. The results of said measurements are shown in FIGS.  15 ″ 17 . 
     FIG. 15 shows the travel speed of ink drops discharged in accordance with the pulse voltages of FIG.  14 . FIG. 16 shows the volume of ink drops discharged in accordance with pulse voltages of FIG.  14 . FIG. 17 shows the printed diameter of ink dots formed from ink drops discharged in accordance with the pulse voltages in FIG.  14 . In these figures, the horizontal axis represents the pulse amplitude of the waveforms shown in FIG. 14, while the vertical axes of FIGS.  15 ″ 17  respectively represent the travel speed, drop volume, and printed dot diameter for each different level of pulse amplitude. 
     Referring to FIG. 15, the travel speed of the ink drops of the large-diameter region stay essentially constant at 5 m/s for waveforms B 4  through B 8 . In contrast, the travel speed of ink drops for waveforms B 1 , B 2 , and B 3  are approximately 30%, 20%, and 10% less, respectively. As shown in FIGS. 16 and 17, as the pulse amplitude of waveforms B 1 -B 8  increases, the discharged ink drop volume and printed ink dot diameter increase. Ink drops formed by waveforms B 1 -B 3  create printed ink dots having approximate diameters of 40 μm, 50 μm, and 60 μm, thus enabling higher image quality than the image created in the first embodiment. 
     It is assumed that printing takes place at 250 dpi for discharged ink drops having travel speeds consistent with the above example. The distance between the carriage  4  and the print sheet  2  is approximately 1 mm. The scan speed of the carriage is 250 mm/s. 
     When the carriage  4  is moved in a forward direction (D 1 ), printing of ink dots of the large-diameter region, corresponding to waveforms B 4 -B 8 , is performed, and the piezoelectric element drive frequency during said printing is 2.5 kHz. When the carriage  4  is moved backward (direction D 2 ), printing of ink dots of the small-diameter region that correspond to waveforms B 1 -B 3  is performed. During printing, the absolute position of carriage  4  within the scan path is detected by means of an encoder (not shown) positioned on carriage  4 . This system prevents a shift in center positions of the printed ink dots. 
     Using such carriage control and printing in this manner, shifts in position for ink dots of the small-diameter region during printing may be prevented and good image quality may be maintained while utilizing a simple print head construction. 
     FIGS.  18 ( a )- 18 ( c ) illustrate an example of ink dots to be printed on a print sheet  2  in accordance with at least the third embodiment. FIG.  18 ( a ) shows printed ink dots that are printed during the forward and backward scanning of the carriage  4 . FIG.  18 ( b ) shows printed ink dots that are printed during the forward scanning of the carriage  4 . FIG.  18 ( c ) shows printed ink dots that are printed during the return scanning of the carriage  4 . Functionally, ink dots of the large-diameter region are printed in a forward pass (FIG.  18 ( b )), and ink dots of the small-diameter region are printed in a return pass (FIG.  12 ( c )). Following the return pass, an image is formed consistent with FIG.  12 ( a ). 
     As described above, an inkjet printer  1  in accordance with the present invention and each of the embodiments prevents ink dot positional shifts for ink dots of the small-diameter region during printing, and good image quality may be maintained while utilizing a simple print head construction. 
     While specific dots are printed when the carriage is moved in a specific direction in the embodiments described above, various combinations of the direction of the carriage movement and the type of printed dots are possible within the scope of the present invention. For Example, although each of the above described embodiments of the present invention discloses a inkjet head, which moves in the main scan direction (i.e., in the traverse direction of the recording sheet), this should not be construed as a limitation. A fixed inkjet head, having a width substantially equal to a width of a recording paper may also be provided where the recording sheet is fed in the vertical direction for scanning, and where the fixed inkjet head includes a plurality of nozzles disposed across the width of the recording sheet. In the inkjet printer employs such fixed type inkjet head, the inkjet head may be controlled so that the inkjet dots corresponding to the small-diameter region are formed during the feeding of the recording sheet in an advanced direction of the vertical direction, and so that the inkjet dots corresponding to the large-diameter region are formed during the feeding of the recording sheet in a reverse direction of the vertical direction. By employing this structure, the inkjet printer can form an excellent image at high speed, inasmuch as the scanning of the inkjet head in the main scan direction is not needed. 
     This application is based on Japanese Patent Application No.09-059262, as filed in Japan, the disclosure of which is incorporated herein by reference. 
     While the invention has been described herein relative to a number of particularized embodiments, it is understood that modifications of, and alternatives to, these embodiments, such modifications and alternatives realizing the advantages and benefits of this invention, will be apparent to those of ordinary skill in the art having reference to this specification and its drawings. It is contemplated that such modifications and alternatives are within the scope of this invention as subsequently claimed herein, and it is intended that the scope of the invention claimed herein be limited only by the broadest interpretation of the appended claims to which the inventors are legally entitled.