Patent Publication Number: US-7591520-B2

Title: Ink jet printer and method for determining pulse width

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to Japanese Patent Application No. 2004-346525, filed on Nov. 30, 2004, the contents of which are hereby incorporated by reference into the present application. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an ink jet printer that applies pulse signals to an actuator of an ink jet head. The present invention further relates to a method for determining pulse width of the pulse signals applied to the actuator of the ink jet head. The ink jet printer of the present invention includes all devices for printing words, images, etc. by discharging ink towards a print medium. For example, the ink jet printer of the present invention includes copying machines, fax machines, multifunctional products, etc. 
     2. Description of the Related Art 
     An ink jet printer has an ink jet head. Usually, the ink jet head has a plurality of units, each unit having a nozzle for discharging ink toward a print medium, a pressure chamber communicating with the nozzle, and an actuator facing the pressure chamber. As one example, a piezoelectric element is used as the actuator. 
     A pulse signal that has at least two levels (high voltage and low voltage) is applied to the piezoelectric element. For example, a pulse signal having a high voltage, this being a base voltage, is applied. The piezoelectric element to which the pulse signal is applied changes voltage in the sequence: high voltage, low voltage, high voltage. When the piezoelectric element changes from high voltage to low voltage, the piezoelectric element deforms away from the pressure chamber. The capacity of the pressure chamber thus increases, and ink is drawn into the pressure chamber. When the piezoelectric element changes from low voltage to high voltage, the piezoelectric element deforms towards the pressure chamber. The capacity of the pressure chamber thus decreases, and pressure of the ink within the pressure chamber is increased. The pressurized ink is discharged from the nozzle. Usually, one ink droplet is discharged from the nozzle when one pulse signal is applied to the piezoelectric element. 
     When one ink droplet is discharged, one dot is formed on the print medium. There are ink jet printers that form one dot on the print medium by continuously discharging a plurality of ink droplets. Pulse signals are applied continuously to the piezoelectric element to continuously discharge a plurality of ink droplets. For example, two ink droplets may be discharged from the nozzle by applying two continuous pulse signals to the piezoelectric element. Usually, the ink droplet which is discharged later has a greater discharge speed than the ink droplet which is discharged first. As a result, the two ink droplets merge before reaching the print medium, and form one ink droplet. When this merged one ink droplet adheres to the print medium, one dot is formed. In this case, the size of the dot is larger than the dot formed from only one ink droplet. As another example, three ink droplets may be discharged from the nozzle by applying three continuous pulse signals to the piezoelectric element. The three ink droplets merge to form one ink droplet. When this merged one ink droplet adheres to the print medium, one dot is formed. In this case, the size of the dot is larger than the dot formed from two ink droplets. 
     In the present specification, a point formed on a print medium by discharging only one ink droplet from a nozzle is termed a dot. Furthermore, a point formed on a print medium by discharging a plurality of ink droplets onto the same location on the print medium from one or a plurality of nozzles is also termed a dot. 
     In the present specification, forming one dot from only one ink droplet is termed single discharging. Forming one dot from two ink droplets is termed double discharging, and forming one dot from three ink droplets is termed triple discharging. 
     The size of the dots can be changed by changing the number of ink droplets used to form one dot. There are ink jet printers which change the size of the dots according to a print mode. 
     Even if the same pulse signals is applied to actuators (for example, piezoelectric elements) that have been manufactured using the same manufacturing process, the ink droplets are not necessarily discharged at the same speed. For example, if the same pulse signals are applied to the piezoelectric element of one ink jet printer and to the piezoelectric elements of another ink jet printer, there may be a difference in the discharge speed of the ink droplets of the former ink jet printer and of the latter ink jet printer. 
     If there is a difference in the discharge speed of the ink droplets between ink jet printers, identical printing results cannot be achieved. A technique for mass-producing ink jet printers that can obtain satisfactory printing results is sought. 
     BRIEF SUMMARY OF THE INVENTION 
     Discharge speed of an ink droplet cannot be known before an ink jet printer is manufactured by assembling each component part. Further, it is known that the discharge speed of the ink droplet varies if the pulse width of the pulse signal applied to the actuator varies. If these issues are taken into account, the mass-production of ink jet printers which can obtain satisfactory printing results is possible by doing the following against each of the ink jet printers. 
     (1) Ink is actually discharged from the ink jet printer, this discharge is observed, and a pulse width of the pulse signal that will obtain satisfactory printing results is determined. 
     The present inventors discovered from their research that the pulse width of pulse signal that can obtain satisfactory printing results may mutually differ in the case of single discharging, double discharging, and triple discharging. 
     Further, the present inventors observed that when one dot was formed utilizing a plurality of continuous pulse signals (for example, double discharging or triple discharging), the manner in which the pulse width of each pulse signal differs may obtain satisfactory printing results. For example, in the case of double discharging, the manner in which the pulse width of the first pulse signal differs from the pulse width of the second pulse signal may obtain satisfactory printing results. Further, in the case of triple discharging, the manner in which the pulse width of the first pulse signal, the pulse width of the second pulse signal, and the pulse width of the third pulse signal mutually differs may obtain satisfactory printing results. 
     Consequently, when a plurality of kinds of pulse signals is utilized, it is preferred that the pulse width of the pulse signals is determined for each kind of pulse signal based on the results of the actual discharge of ink. For example, it is preferred that the pulse width of the pulse signals is determined for each case: the pulse width of single discharging; the first pulse width and the second pulse width of double discharging; and the first pulse width, the second pulse width, and the third pulse width of triple discharging. 
     (2) When the pulse width of each kind of pulse signal is determined, the ink jet printer is set to execute printing by utilizing each determined pulse width. 
     If each ink jet printer is manufactured as described above, various kinds of pulse signals that can obtain satisfactory printing results are applied to the actuator. As a result, ink jet printers that can obtain satisfactory printing results may be manufactured. 
     If a plurality of kinds of pulse signals is utilized, as described above, a plurality of kinds of pulse widths (there are six kinds of pulse widths in the above example) may be obtained. In this case, after the plurality of kinds of pulse widths have been obtained, these must all be input into the ink jet printer, and consequently the inputting operation takes time. The present embodiment teaches a technique for reducing the time required for this inputting operation. 
     The present inventors observed that the pulse widths of the pulse signals utilized by the ink jet printer may be determined by a combination of a base pulse width and a predetermined value. For example, if a base pulse width ‘t’ is multiplied by a predetermined value α, a pulse width (t×α) of a pulse signal may be determined. For example, if a pulse width that can obtain satisfactory printing results is T, the predetermined value a can be determined by dividing T by t. 
     In the case where a plurality of kinds of pulse signals having differing pulse widths is applied to the actuator, the base pulse width may be determined for each of the pulse signals. For example, the base pulse width for the pulse signal for single discharging might be determined as t 1 , the base pulse width for the first pulse signal for double discharging might be determined as t 2 , and the base pulse width for the second pulse signal for double discharging might be determined as t 3 . t 1 , t 2 , and t 2  may be mutually differing values. 
     The present inventors observed that, if each base pulse width for the different kinds of pulse signals is determined in advance, each pulse width for the different kinds of pulse signals may be determined merely by multiplying the base pulse width by one predetermined value. A pulse width T for the pulse signal of single discharging is obtained. This pulse width T can obtain satisfactory printing results. When the obtained pulse width T is divided by the base pulse width t 1 , α 1  is obtained. When α 1  is multiplied by the base pulse width t 1 , the pulse width for single discharging may be obtained. Further, when α 1  is multiplied by the base pulse width t 2 , the pulse width of the first signal for double discharging may be obtained. When α 1  is multiplied by the base pulse width t 3 , the pulse width of the second signal for double discharging may be obtained. The present inventors observed that satisfactory printing results may be achieved by utilizing two pulse widths obtained for double discharging in this manner. That is, when satisfactory printing results can be achieved from a pulse width obtained by multiplying the first kind of base pulse width by the predetermined value, satisfactory printing results may also be achieved from a pulse width obtained by multiplying the second kind of base pulse width by the same value. 
     An ink jet printer taught in the present specification comprises a device for storing base pulse widths corresponding to various kinds of pulse signals. Further, the ink jet printer comprises an inputting device for inputting the predetermined value. For example, a manufacturer or user of the ink jet printer may input the predetermined value to the inputting device. This inputting device includes an interface connected to an external device. For example, the manufacturer or the user may input the predetermined value to the external device. In this case, the predetermined value that has been input to the external device is input to the interface of the ink jet printer. 
     A device for applying the pulse signals to the actuator determines pulse widths of the various kinds of pulse signals by multiplying each kind of base pulse width by the predetermined value. 
     With this ink jet printer, the various pulse widths of the plurality of kinds of pulse signals are set by the manufacturer or the user merely inputting the predetermined value. When this ink jet printer is utilized, the time required for the inputting operation may be made shorter. 
     The above description is merely an example, and the scope of the present invention is not restricted based on the above description. The scope of the present invention is determined on the basis of the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a schematic block diagram of an ink jet printer. 
         FIG. 2  shows a plan view of an ink jet head. 
         FIG. 3  shows an expanded view of a region D of  FIG. 2 . In  FIG. 3 , pressure chambers, apertures, and nozzles are shown by solid lines. 
         FIG. 4  shows a cross-sectional view along the line IV-IV of  FIG. 3 . 
         FIG. 5  shows an expanded plan view of a portion of an actuator unit. 
         FIG. 6  shows a time sequence of changes of a piezoelectric element when one pulse signal is applied to the piezoelectric element.  FIG. 6(A)  shows a state of the piezoelectric element when a high voltage has been applied.  FIG. 6(B)  shows a state of the piezoelectric element when a low voltage has been applied.  FIG. 6(C)  shows a state of the piezoelectric element when a high voltage has again been applied. 
         FIG. 7  shows the circuit configuration of a controller and its surrounds. 
         FIG. 8  shows an example of contents stored in a base timing storage. 
         FIG. 9  shows an example of contents stored in a coefficient storage. 
         FIG. 10(A)  shows base pulse signals for single discharging.  FIG. 10(B)  shows pulse signals for single discharging.  FIG. 10(C)  shows how voltage of the piezoelectric element changes. 
         FIG. 11(A)  shows base pulse signals for double discharging.  FIG. 11(B)  shows pulse signals for double discharging. 
         FIG. 12(A)  shows base pulse signals for triple discharging.  FIG. 12(B)  shows pulse signals for triple discharging. 
         FIG. 13  shows a flowchart of a process of manufacturing the ink jet printer. 
         FIG. 14  shows a graph with pulse width on the horizontal axis and ink droplet discharge speed on the vertical axis. 
         FIG. 15  shows a graph with pulse width on the horizontal axis and ink droplet discharge speed on the vertical axis. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     An applying device may apply a pulse signal for single discharging to an actuator within a predetermined period. In this case, the actuator makes a nozzle discharge one ink droplet to form one dot on a print medium when the pulse signal is applied to the actuator within the predetermined period. 
     In this case, a first storage may store a base pulse width for single discharging and a base pulse width of other pulse signal. The applying device may determine the pulse width of the pulse signal for single discharging by multiplying the base pulse width for single discharging by a predetermined value. Further, the applying device may determine the pulse width of the other pulse signal by multiplying other base pulse width by the predetermined value. 
     The ink jet printer may determine the pulse width for single discharging by utilizing the base pulse width and the predetermined value. 
     The applying device may apply a second pulse signal and a third pulse signal to the actuator within the predetermined period so as to perform double discharging. In this case, the actuator makes the nozzle discharge two ink droplets to form one dot on the print medium when the two pulse signals are applied to the actuator within the predetermined period. 
     The ink jet printer is capable of determining a second pulse width and a third pulse width for double discharging by utilizing the respective base pulse widths and the predetermined value. 
     The first storage may store a base pulse width corresponding with the second pulse signal, a base pulse width corresponding with the third pulse signal, and a first base period between these two pulse signals. In this case, the applying device may determine a period between the two pulse signals by multiplying the first base period stored in the first storage by the predetermined value stored in the second storage. 
     When this is done, the period between the second pulse signal and the third pulse signal for double discharging may be determined by utilizing the base period and the predetermined value. 
     The applying device may apply a fourth pulse signal, a fifth pulse signal and a sixth pulse signal to the actuator within the predetermined period so as to perform triple discharging. In this case, the actuator makes the nozzle discharge three ink droplets to form one dot on the print medium when the three pulse signals are applied to the actuator within the predetermined period. 
     The ink jet printer is capable of determining a fourth pulse width, a fifth pulse width and a sixth pulse width for performing triple discharging by utilizing the respective base pulse widths and the predetermined value. 
     The first storage may store a base pulse width corresponding with the fourth pulse signal, a base pulse width corresponding with the fifth pulse signal, a base pulse width corresponding with the sixth pulse signal, a second base period between the fourth pulse signal and the fifth pulse signal, and a third base period between the fifth pulse signal and the sixth pulse signal. In this case, the applying device may determine a period between the fourth pulse signal and the fifth pulse signal by multiplying the second base period stored in the first storage by the predetermined value stored in the second storage. Further, the applying device may determine a period between the fifth pulse signal and the sixth pulse signal by multiplying the third base period stored in the first storage by the predetermined value stored in the second storage. 
     The ink jet head may further comprise a pressure chamber communicating with the nozzle. The actuator may be a piezoelectric element facing the pressure chamber. 
     The ink jet head may comprise a plurality of units. Each unit may comprise the nozzle, the pressure chamber, and the piezoelectric element. The piezoelectric elements may be divided into a plurality of element groups (these may be termed actuator units). Each element group may comprise a common electrode, a plurality of individual electrodes, and a piezoelectric layer disposed between the common electrode and the individual electrodes. An inputting device may input the predetermined value for each element group. The second storage may store a plurality of combinations of the predetermined value and the element group. The applying device may determine the pulse width of each kind of pulse signal for each element group by multiplying the corresponding base pulse width stored in the first storage by the predetermined value combined with the element group in the second storage. 
     With this configuration, the pulse width of each kind of pulse signal may be set in units of the actuator units. This ink jet printer functions effectively in the case where each actuator unit has a different ink discharging performance when the same pulse signal is applied thereto. 
     Each of the piezoelectric elements may have a different ink discharging performance when the same pulse signal is applied thereto. In this case, the following technique is effective. The inputting device may input the predetermined value for each piezoelectric element. The second storage may store a plurality of combinations of the predetermined value and the piezoelectric element. The applying device determines the pulse width of each kind of pulse signal for each piezoelectric element by multiplying the corresponding base pulse width stored in the first storage by the predetermined value combined with the piezoelectric element in the second storage. 
     When this is done, the pulse width of each kind of pulse signal may be set in units of the piezoelectric elements. 
     If the ink jet printer comprises a plurality of inkjet heads, each of the ink jet heads may have a different ink discharging performance when the same pulse signal is applied thereto. In this case, the following technique is effective. The inputting device may input the predetermined value for each ink jet head. The second storage may store a plurality of combinations of the predetermined value and the ink jet head. The applying device may determine the pulse width of each kind of pulse signal for each ink jet head by multiplying the corresponding base pulse width stored in the first storage by the predetermined value combined with the ink jet head in the second storage. 
     When this is done, the pulse width of each kind of pulse signal may be set in units of the ink jet heads. 
     In the ink jet printer that is utilizing single discharging, the predetermined value that is input by the inputting device may be determined as follows. This method may perform a step of specifying a pulse width of a pulse signal which is capable of obtaining the largest ink droplet discharging speed when the pulse signal is applied to the actuator within the predetermined period. This method may perform a step of dividing the pulse signal specified in the above step by the base pulse width that corresponds with the pulse signal for single discharging. When this is done, the predetermined value may be obtained. 
     The following method is also useful. This method is a method of determining the pulse widths of at least two kinds of pulse signals which are to be applied to an actuator of an ink jet head. The ink jet head comprises a nozzle that discharges an ink droplet toward a print medium, and the actuator that makes the nozzle discharge the ink droplet when the pulse signal is applied to the actuator. The method comprises a step of determining at least two kinds of base pulse widths. Each kind of base pulse width corresponds with a different kind of pulse signal, and each kind of base pulse width mutually differ. Further, this method comprises a step of determining a predetermined value. This method comprises a step of determining a pulse width of each kind of pulse signal by multiplying the corresponding base pulse width by the predetermined value. 
     With this method, the pulse widths of the different kinds of pulse signals may easily be determined. 
     FIRST EMBODIMENT 
     An ink jet printer  1  of a first embodiment will be described with reference to the drawings. Below, the ink jet printer  1  may simply referred to as printer  1 .  FIG. 1  is a schematic block diagram of the printer  1 . 
     The printer  1  has a controller  100 . The controller  100  executes general control of the operation of the printer  1 . Further, the printer  1  has an operation panel  250 . Information can be input using the operation panel  250 . The operation panel  250  is connected with the controller  100 , and the information input to the operation panel  250  is taken to the controller  100 . 
     The printer  1  has a supply device  114 . This supply device  114  has a paper housing section  115 , a paper supply roller  145 , a pair of rollers  118   a  and  118   b , a pair of rollers  119   a  and  119   b , etc. The paper housing section  115  can house a plurality of sheets of printing paper P in a stacked state. The printing paper P has a rectangular shape extending in the left-right direction of  FIG. 1 . The paper supply roller  145  delivers the uppermost sheet of printing paper P in the paper housing section  115  in the direction of the arrow P 1 . The printing paper P that was transported in the direction of the arrow P 1  is then transported in the direction of the arrow P 2  by the pair of rollers  118   a  and  118   b  and the pair of rollers  119   a  and  119   b.    
     The printer  1  has a conveying unit  120 . The conveying unit  120  conveys the printing paper P, that has been transported in the direction of the arrow P 2 , in the direction P 3 . The conveying unit  120  has a belt  111 , belt rollers  106  and  107 , etc. The belt  111  is wound across the belt rollers  106  and  107 . The belt  111  is adjusted to have a length such that a predetermined tension is generated when it is wound across the belt rollers  106  and  107 . The belt  111  has an upper face  111   a  that is located above the belt rollers  106  and  107 , and a lower face  111   b  that is located below the belt rollers  106  and  107 . The first belt roller  106  is connected to a conveying motor  147 . The conveying motor  147  is caused to rotate by the controller  100 . The other belt roller  107  rotates following the rotation of the belt roller  106 . When the belt rollers  106  and  107  rotate, the printing paper P mounted on the upper face  111   a  of the belt  111  is conveyed in the direction shown by the arrow P 3 . 
     A pair of nip rollers  138  and  139  are disposed near the belt roller  107 . The upper nip roller  138  is disposed at an outer peripheral side of the belt  111 . The lower nip roller  139  is disposed at an inner peripheral side of the belt  111 . The belt  111  is gripped between the pair of nip rollers  138  and  139 . The nip roller  138  is energized downwards by a spring (not shown). The nip roller  138  pushes the printing paper P onto the upper face  111   a  of the belt  111 . In the present embodiment, an outer peripheral face of the belt  111  comprises adhesive silicon gum. As a result, the printing paper P adheres reliably to the upper face  111   a  of the belt  111 . 
     A sensor  133  is disposed to the left of the nip roller  138 . The sensor  133  is a light sensor comprising a light emitting element and a light receiving element. The sensor  133  detects a tip of the printing paper P. Detection signals of the sensor  133  are sent to the controller  100 . The controller  100  can determine that the printing paper P has reached a detecting position when the detection signals from the sensor  133  are input. 
     The printer  1  has a head unit  2 . The head unit  2  is located above the conveying unit  120 . The head unit  2  has four ink jet heads  2   a ,  2   b ,  2   c , and  2   d . The ink jet heads  2   a  to  2   d  are all fixed to a printer main body (not shown). The ink jet heads  2   a  to  2   d  have ink discharging faces  13   a  to  13   d  respectively. The ink discharging faces  13   a  to  13   d  are formed at lower faces of the ink jet heads  2   a  to  2   d . Ink is discharged downwards from the ink discharging faces  13   a  to  13   d  of the ink jet heads  2   a  to  2   d . Each ink jet head  2   a  to  2   d  has an approximately rectangular parallelepiped shape that extends in a perpendicular direction relative to the plane of the page of  FIG. 1 . Magenta (M) ink is discharged from the ink jet head  2   a . Yellow (Y) ink is discharged from the ink jet head  2   b . Cyan (C) ink is discharged from the ink jet head  2   c . Black (K) ink is discharged from the ink jet head  2   d . In the present embodiment, four colors of ink can be used to perform color printing of the printing paper P. The configuration of the ink jet heads  2   a  to  2   d  will be described in detail later. The operation of the ink jet heads  2   a  to  2   d  is controlled by the controller  100 . 
     A space is formed between the ink discharging faces  13   a  to  13   d  of the ink jet heads  2   a  to  2   d  and the upper face  111   a  of the belt  111 . The printing paper P is transported towards the left (in the direction of the arrow P 3 ) along this space. Ink is discharged from the ink jet heads  2   a  to  2   d  onto the printing paper P during this process of delivery in the direction of the arrow P 3 . The printing paper P is thus printed with color words or images. In the present embodiment, the ink jet heads  2   a  to  2   d  are fixed. That is, the printer  1  of the present embodiment is a line type printer. 
     A plate  140  is supplied to the left of the conveying unit  120 . When the printing paper P is transported in the direction of the arrow P 3 , a right edge of the plate  140  enters between the printing paper P and the belt  111 , thus separating the printing paper P from the belt  111 . 
     A pair of rollers  121   a  and  121   b  is formed to the left of the plate  140 . Further, a pair of rollers  122   a  and  122   b  is formed above the pair of rollers  121   a  and  121   b . The printing paper P, which has been transported in the direction of the arrow P 3 , is transported in the direction of an arrow P 4  by the pair of rollers  121   a  and  121   b  and the pair of rollers  122   a  and  122   b . A paper discharge section  116  is disposed to the right of the rollers  122   a  and  122   b . The printing paper P that has been transported in the direction of the arrow P 4  is received in the paper discharge section  116 . The paper discharge section  116  can maintain a plurality of printed sheets of printing paper P in a stacked state. 
     Next, the configuration of the ink jet head  2   a  will be described. Since the other ink jet heads  2   b  to  2   d  have the same configuration as the ink jet head  2   a , a detailed description thereof will be omitted. 
       FIG. 2  shows a plan view of the ink jet head  2   a  viewed from an upper side of  FIG. 1 . The ink jet head  2   a  has a passage unit  4  and four actuator units  21   a ,  21   b ,  21   c , and  21   d.    
     Ink passages  5  are formed within the passage unit  4 . In  FIG. 2 , main ink passages  5  within the passage unit  4  are shown by hatching. A plurality of openings  5   a  is formed in an upper face (a face of a proximate side perpendicular to the plane of  FIG. 2 ) of the passage unit  4 . These openings  5   a  are connected to an ink tank (not shown). In the case of the ink jet head  2   a , the openings  5   a  are connected to an ink tank that houses magenta ink. The ink in the ink tank is led into the passage unit  4  via the openings  5   a . The ink discharging face  13   a  is formed at a lower face (a face of a far side perpendicular to the plane of  FIG. 2 ) of the passage unit  4 . 
     The ink passages  5  of the passage unit  4  have ink chambers E 1  to E 4 . The ink chambers E 1  to E 4  are formed in a region that faces the actuator units  21   a  to  21   d . In  FIG. 2 , reference numbers have been applied only to the ink chambers E 1  to E 4  facing the actuator unit  21   b . Actually, however, four ink chambers are also formed in a region facing the actuator unit  21   a , and four ink chambers are formed respectively in regions facing the actuator units  21   c  and  21   d . The ink chambers E 1  to E 4  extend in the up-down direction of  FIG. 2 . The ink chambers E 1  to E 4  are aligned so as to be parallel in the left-right direction of  FIG. 2 . The ink chambers E 1  to E 4  are filled with ink that is introduced from the ink tank via the openings  5   a.    
     The four actuator units  21   a  to  21   d  are fixed to the upper face of the passage unit  4 . The actuator units  21   a  to  21   d  each have a trapezoid shape when viewed from a plan view. The actuator units are aligned in the sequence  21   a ,  21   b ,  21   c , and  21   d  from an upper side of  FIG. 2 . The actuator units  21   a  and  21   c  are disposed such that short edges thereof are at the right side and long edges thereof are at the left side. The actuator units  21   b  and  21   d  are disposed such that short edges thereof are at the left side and long edges thereof are at the right side. The actuator units  21   a  and  21   b  are disposed so as to overlap in the left-right direction of  FIG. 2 . Further, the actuator units  21   a  and  21   b  are disposed so as to overlap in the up-down direction of  FIG. 2 . Similarly, the actuator units  21   b  and  21   c  are disposed so as to overlap in the left-right direction and the up-down direction. The actuator units  21   c  and  21   d  are disposed so as to overlap in the left-right direction and the up-down direction. 
     An FPC (Flexible Printed Circuit: not shown) is connected to the actuator units  21   a  to  21   d . The FPC applies pulse signals (discharge signals) to the actuator units  21   a  to  21   d . The actuator units  21   a  to  21   d  increase or reduce pressure of ink within pressure chambers  10  (to be described: see  FIG. 3 , etc.) of the passage unit  4  in response to the pulse signals. Ink is thus discharged from the passage unit  4 . 
     Below, unless otherwise specified, the actuator units  21   a  to  21   d  are represented as the reference number  21 . 
       FIG. 3  is an expanded plan view of a region D of  FIG. 2 . In  FIG. 3 , nozzles  8 , pressure chambers  10 , and apertures  12  which actually cannot be seen are shown by solid lines. 
     As shown in  FIG. 3 , a plurality of nozzles  8 , a plurality of pressure chambers  10  and a plurality of apertures  12 , etc. are formed within the passage unit  4 . The number of nozzles  8 , of pressure chambers  10 , and of apertures  12  is identical. In  FIG. 3 , not all the nozzles  8 , pressure chambers  10 , and apertures  12  are numbered. 
     The actuator unit  21  has a plurality of individual electrodes  35 . One individual electrode  35  corresponds to one pressure chamber  10 . The number of individual electrodes  35  is identical with the number of pressure chambers  10 . 
     The configuration of the passage unit  4  and the actuator unit  21  will be described in detail with reference to  FIG. 4 .  FIG. 4  is a cross-sectional view along the line IV-IV of  FIG. 3 . 
     The passage unit  4  is a structure in which nine metal plates  22  to  30  have been stacked. The nozzles  8  are formed in a nozzle plate  30 , and pass through this nozzle plate  30 . Only one nozzle  8  is shown in  FIG. 4 . However, a plurality of nozzles  8  is actually formed (see  FIG. 3 ). 
     A cover plate  29  is stacked on a surface of the nozzle plate  30 . A plurality of through holes  29   a  is formed in the cover plate  29 . The through holes  29   a  are formed in positions corresponding to the nozzles  8  of the nozzle plate  30 . 
     Three manifold plates  26 ,  27 , and  28  are stacked on a surface of the cover plate  29 . A through hole  26   a  is formed in the manifold plate  26 . A through hole  27   a  is formed in the manifold plate  27 , and a through hole  28   a  is formed in the manifold plate  28 . The through holes  26   a ,  27   a , and  28   a  are formed in a position corresponding to the through hole  29   a  of the cover plate  29 . The manifold plates  26 ,  27 , and  28  have long holes  26   b ,  27   b , and  28   b  respectively. The long holes  26   b ,  27   b , and  28   b  have the shape of the ink passages  5  shown in  FIGS. 2 and 3 . The long holes  26   b ,  27   b , and  28   b  are each formed in the same position. Spaces formed by the long holes  26   b ,  27   b , and  28   b  are the ink passages  5 . In  FIG. 4 , the ink chamber E 1 , which is a part of the ink passage  5 , is shown. 
     A supply plate  25  is stacked on a surface of the manifold plate  26 . A through hole  25   a  is formed in the supply plate  25 . The through hole  25   a  is formed in a position corresponding to the through hole  26   a  of the manifold plate  26 . Further, a through hole  25   b  is formed in the supply plate  25 . The through hole  25   b  is formed in a position corresponding to the long hole  26   b  of the manifold plate  26 . 
     An aperture plate  24  is stacked on a surface of the supply plate  25 . A through hole  24   a  is formed in the aperture plate  24 . The through hole  24   a  is formed in a position corresponding to the through hole  25   a  of the supply plate  25 . Further, a long hole  24   b  is formed in the aperture plate  24 . Right edge of the long hole  24   b  is formed in a position corresponding to the through hole  25   b  of the supply plate  25 . The long hole  24   b  functions as the aperture  12 . 
     A base plate  23  is stacked on a surface of the aperture plate  24 . A through hole  23   a  is formed in the base plate  23 . The through hole  23   a  is formed in a position corresponding to the through hole  24   a  of the aperture plate  24 . Further, a through hole  23   b  is formed in the base plate  23 . The through hole  23   b  is formed in a position corresponding to left edge of the long hole  24   b  of the aperture plate  24 . 
     A cavity plate  22  is stacked on a surface of the base plate  23 . A long hole  22   a  is formed in the cavity plate  22 . Left edge of the long hole  22   a  is formed in a position corresponding to the through hole  23   a  of the base plate  23 . Right edge of the long hole  22   a  is formed in a position corresponding to the through hole  23   b  of the base plate  23 . The long hole  22   a  functions as the pressure chamber  10 . The pressure chamber  10  communicates with the ink chamber E 1  via the through hole  23   b , the aperture  12 , and the through hole  25   b . Further, the pressure chamber  10  communicates with the nozzle  8  via the through hole  23   a , the through hole  24   a , the through hole  25   a , the through hole  26   a , the through hole  27   a , the through hole  28   a , and the through hole  29   a.    
     As shown in  FIG. 3 , the pressure chambers  10  are substantially diamond shaped when viewed from a plan view. The plurality of pressure chambers  10  is disposed in a staggered pattern. One pressure chamber row is formed by aligning a plurality of the pressure chambers  10  in a direction orthogonal to the direction of the arrow P 3  (the left-right direction of  FIG. 3 ). Sixteen pressure chamber rows are aligned in the direction of P 3  within a region corresponding to one actuator unit  21 . Each pressure chamber  10  communicates with one out of the ink chambers E 1  to E 4 . 
     One nozzle row is formed by aligning a plurality of the nozzles  8  in a direction orthogonal to the direction of the arrow P 3 . Sixteen nozzle rows are aligned in the direction of P 3  within a region corresponding to one actuator unit  21 . Each nozzle  8  communicates with one out of the pressure chambers  10 . As shown in  FIG. 3 , when the ink jet head  2  is viewed from a plan view, none of the nozzles  8  overlap with the ink chambers E 1  to E 4 . 
     The nozzles  8  are mutually offset in the direction orthogonal to the direction of the arrow P 3 . That is, if the nozzles  8  are projected from the direction of P 3  on a straight line (a projective line) extending in the direction orthogonal to the arrow P 3 , each nozzle  8  will be present at differing position on this projective line. Each nozzle  8  on the projective line is separated from an adjacent nozzle  8  with uniform space. This space is a distance corresponding to 600 dpi. This 600 dpi is the resolution in the direction orthogonal to the arrow P 3 . 
     Returning to  FIG. 4 , the configuration of the actuator unit  21  will be described. The actuator unit  21  is connected to the surface of the cavity plate  22 . Actually, the four actuator units  21   a  to  21   d  are connected to the cavity plate  22 . 
     The actuator unit  21  comprises four piezoelectric sheets  41 ,  42 ,  43 , and  44 , a common electrode  34 , the individual electrodes  35 , etc. The thickness of each of the piezoelectric sheets  41  to  44  is approximately 15 μm. The thickness of the actuator unit  21  is approximately 60 μm. Each of the piezoelectric sheets  41  to  44  has approximately the same area as the one actuator unit  21  shown in  FIGS. 2 and 3 . That is, the piezoelectric sheets  41  to  44  each have a trapezoid shape when viewed from a plan view. The piezoelectric sheets  41  to  44  extend across the plurality of pressure chambers  10 . The piezoelectric sheets  41  to  44  are formed from ferroelectric lead zirconate titanate (PZT) ceramic material. 
     The common electrode  34  is disposed between the uppermost piezoelectric sheet  41  and the piezoelectric sheet  42  formed below the piezoelectric sheet  41 . The common electrode  34  has approximately the same area as the piezoelectric sheets  41  to  44 , and has a trapezoid shape when viewed from a plan view. The common electrode  34  has a thickness of approximately 2 μm. The common electrode  34  is made from a metal material such as, for example, Ag—Pd. Electrodes are not disposed between the piezoelectric sheet  42  and the piezoelectric sheet  43 , between the piezoelectric sheet  43  and the piezoelectric sheet  44 , or between the piezoelectric sheet  44  and the cavity plate  22 . The common electrode  34  is connected with a ground (not shown). 
     A plurality of the individual electrodes  35  is disposed on the surface of the uppermost piezoelectric sheet  41 . Each individual electrode  35  has a thickness of 1 μM. Each individual electrode  35  is disposed in a position corresponding to different one of the pressure chambers  10 . The individual electrodes  35  are made from a metal material such as, for example, Ag—Pd. A land  36  having a thickness of approximately 15 μm is formed at one end of each individual electrode  35 . The lands  36  are substantially circular when viewed from a plan view, and the diameter thereof is approximately 160 μm. The individual electrode  35  and the land  36  are joined conductively. The lands  36  may be composed of, for example, metal that contains glass flit. The land  36  is electrically connected with the individual electrode  35  and with a contact formed on the FPC (not shown). The individual electrode  35  is electrically connected with a driver IC  220  (to be described; see  FIG. 7 ) via the contact and wiring of the FPC. The driver IC  220  is controlled by the controller  100 . The controller  100  can thus individually control the voltage of each of the individual electrodes  35 . 
       FIG. 5  shows an expanded plan view of a portion of the actuator unit  21 . As shown in  FIG. 5 , each of the individual electrodes  35  is substantially diamond shaped when viewed from a plan view. One individual electrode  35  faces one pressure chamber  10 . The individual electrode  35  is smaller than the pressure chamber  10 . The major part of the individual electrode  35  overlaps with the pressure chamber  10 . A protruding part  35   a  is formed on each individual electrode  35 . This protruding part  35   a  extends downwards from an acute angle of a lower side of the diamond shape. The protruding part  35   a  extends into a region  41   a  in which the pressure chambers  10  are not formed. The lands  36  are formed in this region  41   a.    
     Since one individual electrode  35  faces one pressure chamber  10 , the individual electrodes  35  are disposed with the same pattern as the pattern with which the pressure chambers  10  are disposed. That is, the plurality of individual electrodes  35  forms electrode rows that are aligned in the direction orthogonal to the arrow P 3 . Sixteen electrode rows are aligned in the direction of the arrow P 3  within one actuator unit  21 . 
     In the present embodiment, the individual electrodes  35  are formed only on the surface of the actuator unit  21 . As will be described in detail later, only the piezoelectric sheet  41  between the common electrode  34  and the individual electrodes  35  forms an activated part of the piezoelectric sheets. With this type of configuration, the unimorph deformation in the actuator unit  21  has superior deformation efficiency. 
     When a voltage difference is applied between the common electrode  34  and the individual electrodes  35 , a region of the piezoelectric sheet  41  to which the electric field is applied deforms due to piezoelectric effects. The deformation part functions as an active part. The piezoelectric sheet  41  can expand and contract in its direction of thickness (the stacking direction of the actuator unit  21 ) and in its planer direction. The other piezoelectric sheets  42  to  44  are non-active layers that are not located between the individual electrodes  35  and the common electrode  34 . Consequently, they cannot deform spontaneously even when a voltage difference is applied between the individual electrodes  35  and the common electrode  34 . In the actuator unit  21 , the upper piezoelectric sheet  41  that is farther from the pressure chambers  10  is the active part, and the lower piezoelectric sheets  42  to  44  that are closer to the pressure chambers  10  are non-active parts. This type of actuator unit  21  is termed a unimorph type. 
     When voltage difference is applied between the common electrode  34  and the individual electrodes  35  such that the direction of the electric field and the direction of polarization have the same direction, the active part of the piezoelectric sheet  41  contracts in a planar direction. By contrast, the piezoelectric sheets  42  to  44  do not contract. There is thus a difference in the rate of contraction of the piezoelectric sheet  41  and the piezoelectric sheets  42  to  44 . As a result, the piezoelectric sheets  41  to  44  (including the individual electrodes  35 ) deform so as to protrude towards the pressure chamber  10  side. The pressure in the pressure chambers  10  is thus increased. By contrast, when there is zero voltage difference between the common electrode  34  and the individual electrodes  35 , the state wherein the piezoelectric sheets  41  to  44  protrude towards the pressure chamber  10  side is released. The pressure in the pressure chambers  10  is thus decreased. 
     The voltage of the individual electrodes  35  is controlled individually. There is deformation of the parts of the piezoelectric sheets  41  to  44  facing the individual electrodes  35  in which the voltage has been changed. One piezoelectric element  20  (see  FIG. 4 ) is formed from one individual electrode  35  and the region facing that individual electrode  35  (the region of the piezoelectric sheets  41  to  44  (i.e. the common electrode  35 )). Only one piezoelectric element  20  has been shown in  FIG. 4 . However, there is the same number of piezoelectric elements  20  as the number of individual electrodes  35  (the same number as the number of pressure chambers  10 ). The piezoelectric elements  20  are disposed with the same pattern as the pattern with which the individual electrodes  35  are disposed. That is, one element row is formed from a plurality of the piezoelectric elements  20  that are aligned in the direction of P 3 . Sixteen element rows are aligned in the direction of P 3  within one actuator unit  21 . The voltage of each piezoelectric element  20  is controlled individually by the controller  100 . 
     The operation of the ink jet head  2  configured as described above will be described with reference to  FIG. 6(A)  to (C). A pulse signal S is applied to the piezoelectric element  20  (the individual electrode  35 ) corresponding to the nozzle  8  so as to discharge an ink droplet from that nozzle  8 . 
     When printing is not being performed, a voltage higher than the voltage of the common electrode  34  is maintained in the individual electrode  35  (the region X of the pulse signal in  FIG. 6(A) ). In this state, the piezoelectric element  20  protrudes towards the pressure chamber  10  side (see  FIG. 6(A) ). 
     The individual electrode  35  of the piezoelectric element  20  is made to have the same voltage as the common electrode  34  (the region Y of the pulse signal in  FIG. 6(B) ). The piezoelectric element  20  thus deforms upwards relative to  FIG. 6 , and the pressure in the pressure chamber  10  is decreased. In this state, the piezoelectric element  20  is the state shown in  FIG. 6  (B). When the pressure in the pressure chamber  10  decreases, the ink in the ink chamber E 1  is led into the pressure chamber  10  via the aperture  12 . The pressure chamber  10  is thus filled with ink. 
     Next, the individual electrode  35  of the piezoelectric element  20  is returned to high voltage (the region Z of the pulse signal in  FIG. 6(C) ). The piezoelectric element  20  deforms downwards, and the pressure in the pressure chamber  10  increases. The ink in the pressure chamber  10  is thus pressurized. One ink droplet is thus discharged from the nozzle  8 . When one ink droplet adheres to the printing paper P, one dot is formed. 
     As described above, in order to discharge one ink droplet from the nozzle  8 , a pulse signal in which a high voltage is the standard is applied to the piezoelectric element  20 . The technique of the present embodiment is termed ‘fill before fire’. If a pulse width of the pulse signal (i.e. the period of the region Y in  FIG. 6(B) ) is set to the time taken for a pressure wave to be proceeded from the nozzle  8  to an opening of the aperture  12  (the left edge in  FIG. 6(A)  etc.), the discharge speed of the ink droplet will be at its maximum. 
     As described above, one dot may be formed by discharging one ink droplet from the nozzle  8 . This is termed single discharging. 
     In the present embodiment, one dot may be formed by continuously discharging two ink droplets from the nozzle  8 . This is termed double discharging. In the case of double discharging, two pulse signals are applied continuously to the piezoelectric element  20 . In this case, the deformation of the piezoelectric element  20  as shown in  FIGS. 6(A)  to(C) is performed twice. Two ink droplets are thus continuously discharged from the nozzle  8 . Usually, the second of these ink droplets has a faster discharge speed than the first of these ink droplets. As a result, the two ink droplets merge before reaching the printing paper P, and form one ink droplet. When this one ink droplet adheres to the printing paper P, one dot is formed. This dot is larger than a dot formed by the single discharging. 
     Further, in the present embodiment, one dot may be formed by continuously discharging three ink droplets from the nozzle  8 . This is termed triple discharging. In the case of triple discharging, three pulse signals are applied continuously to the piezoelectric element  20 . In this case, three ink droplets are thus continuously discharged from the nozzle  8 . The three ink droplets merge before reaching the printing paper P, and form one ink droplet. When this one ink droplet adheres to the printing paper P, one dot is formed. This dot is larger than a dot formed by the double discharging. 
     The user of the printer  1  may select either of two printing modes. When the user selects printing mode  1 , the printer  1  performs printing using only single discharging. When the user selects printing mode  2 , the printer  1  performs printing using a mixture of single discharging, double discharging and triple discharging. That is, the dots are formed on one sheet of printing paper P utilizing all of single discharging, double discharging and triple discharging. Dots of differing sizes are therefore formed on one sheet of printing paper P. In this case, there is a richer graduation than in the case of the printing mode  1 . 
     Next, the configuration of the controller  100  for controlling the ink jet heads  2   a  to  2   d  will be described. The controller  100  prints on the printing paper P by causing ink to be discharged from the nozzles  8  while moving the printing paper P in the direction of the arrow P 3 . 
       FIG. 7  is a block view showing the functions of the controller  100 . The controller  100  comprises a CPU (Central Processing Unit), a ROM (Read Only Memory), a RAM (Random Access Memory), etc. Each section in  FIG. 7  is constructed by performing these functions. The CPU is a processing unit. The CPU executes programs stored in the ROM. The ROM stores programs to be executed by the CPU, and stores data used in the execution of these programs. The RAM temporarily stores data. 
     The controller  100  comprises a print data storage  200 , a base timing storage  202 , a coefficient storage  204 , a print signal creating portion  206 , a movement controller  208 , an inputting portion  210 , and an outputting portion  212 , etc. 
     The print data storage  200  stores print data output from a PC  252 . The print data will be described later. Furthermore, the print data storage  200  can store the printing mode selected by the user. 
     The base timing storage  202  stores the timing of rises and falls of base pulse signals.  FIG. 8  schematically shows contents stored in the base timing storage  202 . In  FIG. 8 , (S) corresponds to single discharging, (D) corresponds to double discharging, and (T) corresponds to triple discharging. The base timing storage  202  stores the base pulse signals for single discharging, for double discharging, and for triple discharging. 
     For single discharging, the base timing storage  202  stores TS 0  to TS 3 . In the case where TS 0  is zero, the base timing storage  202  stores ‘a fall time TS 1 , a rise time TS 2 , and one printing period ending time TS 3 .’ The difference between the time TS 1  and the time TS 2  is a pulse width WS of the base pulse signal for single discharging. 
     For double discharging, the base timing storage  202  stores TD 0  to TD 5 . In the case where TD 0  is zero, the base timing storage  202  stores ‘a first fall time TD 1 , a first rise time TD 2 , a second fall time TD 3 , a second rise time TD 4 , and one printing period ending time TD 5 .’ The difference between the time TD 1  and the time TD 2  is a first pulse width WD 1  of the base pulse signal for double discharging. The difference between the time TD 3  and the time TD 4  is a second pulse width WD 2  of the base pulse signal for double discharging. In the present embodiment, the time between TD 2  and TD 3  is identical with the time between TD 1  and TD 2  (i.e. WD 1 ). TS 3  and TD 5  are identical. 
     For triple discharging, the base timing storage  202  stores TT 0  to TT 7 . In the case where TT 0  is zero, the base timing storage  202  stores ‘a first fall time TT 1 , a first rise time TT 2 , a second fall time TT 3 , a second rise time TT 4 , a third fall time TT 5 , a third rise time TT 6 , and one printing period ending time TD 7 .’ The difference between the time TT 1  and the time TT 2  is a first pulse width WT 1  of the base pulse signal for triple discharging. The difference between the time TT 3  and the time TT 4  is a second pulse width WT 2  of the base pulse signal for triple discharging. The difference between the time TT 5  and the time TT 6  is a third pulse width WT 3  of the base pulse signal for triple discharging. In the present embodiment, the time between TT 2  and TT 3  is identical with the time between TT 1  and TT 2  (i.e. WT 1 ). Further, the time between TT 4  and TT 5  is identical with the time between TT 3  and TT 4  (i.e. WT 2 ). TT 7 , TS 3  and TD 5  are identical. 
     The manner in which the base pulse signals are obtained will be described in detail later. 
     The coefficient storage  204  stores coefficients for each of the actuator units  21 .  FIG. 9  shows a simplification of contents stored in the coefficient storage  204 . The coefficient storage  204  stores a plurality of combinations of one actuator unit  21  and one coefficient. The printer  1  of the present embodiment has four ink jet heads  2   a  to  2   d , and four actuator units  21   a  to  21   d  are present for each of the ink jet heads  2   a  etc. As a result, there are sixteen actuator units  21 . The coefficient storage  204  stores the coefficients for each of the sixteen actuator units  21 . That is, sixteen coefficients α 1  to α 16  are stored. 
     The manner in which the coefficients are determined will be described in detail later. Further, the manner in which the coefficients are utilized will be described next. 
     The print signal creating portion  206  of  FIG. 7  creates print signals based on the print data stored in the print data storage  200  and on the printing mode. The print data has been output from the PC  252 . The print data includes information showing the coordinate and color of a dot to be formed on the printing paper P. The printing mode has been input by the user. The print signal is data showing which pulse signal (single, double, or triple) should be applied to which piezoelectric element  20  with which timing. 
     For example, the print data includes information showing that a dot should be formed at a coordinate (xA, yB). The print signal creating portion  206  can specify the piezoelectric element  20  (in this case  20 A) for forming the dot at the coordinate (xA, yB). 
     As described above, TS 3 , TD 5 , and TT 7  (see  FIG. 8 ) are identical in the present embodiment. That is, the time (this is termed the printing period) for forming one dot is identical for single discharging, double discharging, and triple discharging. As a result, printing can be performed using all of single discharging, double discharging, and triple discharging within one printing period. In this case, the dots formed within one printing period may include dots formed by single discharging, dots formed by double discharging, and dots formed by triple discharging. The printing period is executed repeatedly while the printing paper P is being moved in the direction P 3  (see  FIG. 1 , etc.). Dots can thus be formed at all coordinates on the printing paper P. 
     In order to form the dot at the coordinate (xA, yB), the print signal creating portion  206  specifies the printing period in which the pulse signal should be applied to the piezoelectric element  20 A. In this example, this is a printing period B. 
     Based on the printing mode, the print signal creating portion  206  determines the size of the dot (i.e. single discharging, double discharging, or triple discharging) to be formed at the coordinate (xA, yB). 
     The piezoelectric element to which the pulse signal should be applied ( 20 A), and the printing period (B), the number of pulse signals (single, double, or triple) is specified by the process executed up to this point. 
     The print signal creating portion  206  specifies the time at which the pulse signal rises and falls corresponding to the number of pulse signals. This process is executed as follows. For example, in the case of single discharging, TS 1  and TS 2  for single discharging (see  FIG. 8 ) are read from the base timing storage  202 . Further, the coefficient of the actuator unit  21  that has the piezoelectric element  20 A (here, this coefficient is α 1 ) is read from the coefficient storage  204 . Then TS 1  and TS 2  are each multiplied by the coefficient that has been read. In the case of the example, α 1 ×TS 1  and α 1 ×TS 2  are obtained. TS 3  is not multiplied by the coefficient. That is, the printing period is fixed. 
     As another example, in the case of double discharging, TD 1 , TD 2 , TD 3  and TD 4  (see  FIG. 8 ) for double discharging are read from the base timing storage  202 . Then each is multiplied by the coefficient. In the case of the example, α 1 ×TD 1 , α 1 ×TD 2 , α 1 ×TD 3 , and α 1 ×TD 4  are obtained. TD 5  is not multiplied by the coefficient. 
     As yet another example, in the case of triple discharging, TT 1 , TT 2 , TT 3 , TT 4 , TT 5 , and TT 6  (see  FIG. 8 ) are read from the base timing storage  202 . Then each is multiplied by the coefficient. In the case of the example, α 1 ×TT 1 , α 1 ×TT 2 , α 1 ×TT 3 , α 1 ×TT 4 , α 1 ×TT 5 , and α 1 ×TT 6  are obtained. TT 7  is not multiplied by the coefficient. 
     The print signal creating portion  206  can create the information for forming one dot by going through the above processes. That is, the print signal creating portion  206  can create the information (the print signal) having the combination of the piezoelectric element to which the pulse signal should be applied (for example,  20 A), the printing period (B), and the timing with which the pulse signal rises and falls (for example, α 1 ×TS 1  and α 1 ×TS 2 ). The print signal creating portion  206  creates the aforementioned information for all the dots to be formed on the printing paper P. The print signal created by the print signal creating portion  206  is output to the corresponding driver IC  220  via the outputting portion  212 . 
     The movement controller  208  controls the conveying motor  147  (see  FIG. 1 ). The printing paper P on the belt  111  is thus conveyed. In the present embodiment, the speed with which discharged printing paper P on the belt  111  is conveyed is constant. Further, the movement controller  208  controls a motor for driving the paper supply roller  145  (see  FIG. 1 ), and controls a motor for driving the rollers  118   a ,  118   b ,  119   a ,  119   b ,  121   a ,  121   b ,  122   a , and  122   b.    
     The PC  252 , the operation panel  250  (see  FIG. 1 ), and the sensor  133  (see  FIG. 1 ) are connected with the inputting portion  210 . The PC  252  converts an image that has been instructed by the user into print data. The print data is data showing the coordinate at which the dot should be formed and the color of that dot. The PC  252  outputs the print data to the printer  1 . The print data output from the PC  252  is input to the inputting portion  210 . The print data that has been input to the inputting portion  210  is stored in the print data storage  200 . 
     Information is input using the operation panel  250 . For example, the user can select the printing mode utilizing the operation panel  250 . The printing mode input by the user is stored in the print data storage  200 . As another example, the manufacturer of the printer  1  can input the coefficients utilizing the operation panel  250 . The coefficients that have been input are stored in the coefficient storage  204 . 
     The sensor  133  outputs detection signals when the sensor  133  detects a tip of the printing paper P. The detection signals are input to the inputting portion  210 . The controller  100  can determine the timing with which the pulse signals are applied to the piezoelectric elements  20  based on the detection signals input to the inputting portion  210 . That is, the timing at which the first printing period should be started can be determined. 
     The outputting portion  212  is connected with the driver ICs  220 . One driver IC  220  is prepared against one actuator unit. In  FIG. 7 , only four actuator units  21   a  to  21   d  of one ink jet head (for example  2   a ) and only four driver ICs  220  are shown. However, sixteen actuator units  21  and sixteen driver ICs  220  are actually present. The driver IC  220  inputs the print signals of serial type output from the controller  100 . The driver IC  220  converts the serial type print signals into parallel type print signals, and amplifies the parallel type print signals. The driver IC  220  provides the parallel type print signals to the actuator units  21 . The driver IC  220  is connected with each piezoelectric element  20  of the corresponding actuator unit  21 . 
     The driver IC  220  creates pulse signals based on the information included in the print signals. For example, in the case where the print data includes the information having the combination of the piezoelectric element  20 A, the printing period B, and ‘α 1 ×TS 1  and α 1 ×TS 2 ’, a pulse signal is created: this pulse signal falls at the timing α 1 ×TS 1  and rises at the timing α 1 ×TS 2 . Thereupon, the pulse signal that has been created is applied to the piezoelectric element  20 A at the printing period B. In this case, the piezoelectric element  20 A deforms for single discharging at the printing period B. 
     As another example, in the case where the print data includes the information having the combination of the piezoelectric element  20 A, the printing period B, and ‘α 1 ×TD 1 , α 1 ×TD 2 , α 1 ×TD 3 , and α 1 ×TD 4 ’, a first pulse signal and a second pulse signal is created: this first pulse signal falls at the timing α 1 ×TD 1  and rises at the timing α 1 ×TD 2 , and this second pulse signal falls at the timing α 1 ×TD 3  and the pulse signal rises at the timing α 1 ×TD 4 . The two pulse signals that have been created are applied to the piezoelectric element  20 A at the printing period B. In this case, the piezoelectric element  20 A deforms for double discharging. 
     As yet another example, in the case where the print data includes the information having the combination of the piezoelectric element  20 A, the printing period B, and ‘α 1 ×TT 1 , α 1 ×TT 2 , α 1 ×TT 3 , α 1 ×TT 4 , α 1 ×TT 5 , and α 1 ×TT 6 ’ a first pulse signal, a second pulse signal, and a third pulse signal are created: this first pulse signal falls at the timing α 1 ×TT 1  and rises at the timing α 1 ×TT 2 , this second pulse signal falls at the timing α 1 ×TT 3  and rises at the timing α 1 ×TT 4 , and this third pulse signal falls at the timing α 1 ×TT 5  and rises at the timing α 1 ×TT 6 . The three pulse signals that have been created are applied to the piezoelectric element  20 A at the printing period B. In this case, the piezoelectric element  20 A deforms for triple discharging. 
       FIG. 10(A)  shows waveforms of the base pulse signal for single discharging. The base pulse signal can be obtained from the contents stored in the base timing storage  202 . 
       FIG. 10(B)  shows pulse signals obtained by multiplying the base pulse signal of  FIG. 10(A)  by the coefficient α 1 . The time at which the pulse signal falls is α 1 ×TS 1 , and the time at which the pulse signal rises is α 1 ×TS 2 . The pulse width of this pulse signal is the value α 1 ×WS obtained by multiplying the base pulse signal WS by α 1 . The ending time of the printing period is fixed at TS 3 . 
       FIG. 10(C)  shows changes in the voltage of the piezoelectric element  20  to which the pulse signal of  FIG. 10(B)  has been applied. The piezoelectric element  20  forms a condenser due to the individual electrodes  35 , the common electrode  34 , and the piezoelectric sheet  41  (see  FIG. 4 ). As a result, the voltage of the piezoelectric element  20  changes somewhat more slowly than the pulse signal. The period for the voltage of the piezoelectric element  20  to rise after it has fallen is the same as the pulse width α 1 ×WS of  FIG. 10(B) . 
       FIG. 11(A)  shows waveforms of the base pulse signals for double discharging. The pulse width of the first base pulse is WD 1 . The pulse width of the second base pulse is WD 2 . A period between the first base pulse and the second base pulse is set to be WD 1 . 
       FIG. 11(B)  shows pulse signals obtained by multiplying the base pulse signals of  FIG. 11(A)  by the coefficient α 1 . The pulse width of the first pulse signal is α 1 ×WD 1 , and the pulse width of the second pulse signal is α 1 ×WD 2 . A period between the first pulse signal and the second pulse signal is α 1 ×WD 1 . The ending time of the printing period is fixed at TD 5 . Moreover, TD 5  is identical with TS 3  (see  FIG. 10 ). 
       FIG. 12(A)  shows waveforms of the base pulse signals for triple discharging. The pulse width of the first base pulse is WT 1 . The pulse width of the second base pulse is WT 2 . The pulse width of the third base pulse is WT 3 . A period between the first base pulse and the second base pulse is set to be WT 1 . A period between the second base pulse and the third base pulse is set to be WT 2 . 
       FIG. 12(B)  shows pulse signals obtained by multiplying the base pulse signals of  FIG. 12(A)  by the coefficient α 1 . The pulse width of the first pulse signal is α 1 ×WT 1 , and the pulse width of the second pulse signal is α 1 ×WT 2 . The pulse width of the third pulse signal is α 1 ×WT 3 . A period between the first pulse signal and the second pulse signal is α 1 ×WT 1 . A period between the second pulse signal and the third pulse signal is α 1 ×WT 2 . The ending time of the printing period is fixed at TT 7 . Moreover, TT 7  is identical with TS 3  (see  FIG. 10 ). That is, TT 7 , TS 3  and TD 5  are identical. 
     The printer  1  of the present embodiment determines the pulse signals to be applied to the piezoelectric elements  20  based on the base pulse signals and each coefficient that has been set for each actuator unit  21 . For example, a pulse signal that was obtained by multiplying the base pulse signal by the coefficient α 1  is applied to the piezoelectric elements  20  of the actuator unit  21  that corresponds to the coefficient α 1 . As another example, a pulse signal that was obtained by multiplying the base pulse signal by the coefficient α 2  is applied to the piezoelectric elements  20  of the actuator unit  21  that corresponds to the coefficient α 2 . 
     The same coefficient can be utilized for the same actuator unit  21  even when the pulse signals that are being applied are for single discharging, double discharging, and for triple discharging. 
     Next, a method of manufacturing the printer  1  will be described. That is, the processes will be described for determining the base pulse signals and the coefficients.  FIG. 13  shows a flowchart of the method of manufacturing the printer  1 . 
     As shown in  FIG. 13 , a base actuator unit is first determined (S 2 ). This process is executed as follows. 
     (S 2 -1) An ideal value AL (Acoustic length) for a pulse width for single discharging is obtained. This value allows maximum discharge speed of the ink droplet in the case of single discharging. AL is a time taken for a pressure wave—this being generated by moving from the state in  FIG. 6(A)  to the state in FIG.  6 (B)—to be proceeded from the nozzle  8  to the opening of the aperture  12  (the left edge of the aperture  12  in  FIG. 6(A) . AL can be calculated from the structure of the ink jet head. 
     (S 2 -2) Next, a pulse signal (for single discharging) having a predetermined pulse width (for example, W 1 ) is applied to a plurality of piezoelectric elements of one actuator unit. The discharge speed of ink droplets discharged from the nozzles is measured. The average value of the measured discharge speed is calculated. 
     (S 2 -3) The process of (S 2 -2) is executed with varying pulse widths. The average value of the ink droplet discharge speed for each pulse width is calculated. 
     (S 2 -4) The results obtained in (S 2 -2) and (S 2 -3) are plotted in a graph in which pulse width is on the horizontal axis and discharge speed is on the vertical axis. Then a curved line is drawn passing through the points that have been plotted. The curved line RO in  FIG. 14  is an example of a curved line obtained by this process. When the curved line is drawn, the pulse width AL 0  in which the maximum discharge speed can be obtained is specified. 
     (S 2 -5) The processes of (S 2 -2) to (S 2 -4) are executed for a plurality of actuator units (for example, for ten actuator units). In this manner, for example ten pulse widths AL 0  are specified. 
     (S 2 -6) An actuator unit is specified from the actuator units for which the processes of (S 2 -2) to (S 2 -5) have been executed: this specified actuator has the pulse width AL 0  which is the closest to the ideal value AL obtained in (S 2 -1). The specified actuator unit becomes the base actuator unit. 
     When the base actuator unit has been specified in S 2  of  FIG. 13 , the base pulse signals are specified based on this actuator unit (S 4 ). That is, TS 0  to TS 3 , TD 0  to TD 5 , and TT 0  to TT 7  of  FIG. 8  are determined. This process is executed as follows. 
     (S 4 -1) First, the base pulse signal for single discharging is specified. Specifically, TS 0  to TS 3  are specified. TS 0  is zero. TS 1  is a value that is half of AL 0  of the base actuator unit. TS 2  is a value where the pulse width AL 0  has been added to TS 1 . The time AL 0  between TS 1  and TS 2  is the pulse width. This pulse width AL 0  becomes the base pulse width WS of  FIG. 10(A) . A predetermined fixed value is utilized as TS 3 . 
     (S 4 -2) The base pulse signals for double discharging are specified. Specifically, TD 0  to TD 5  of  FIG. 8  are specified. This process is executed as follows. 
     (S 4 -2-1) Pulse signals for double discharging are applied to the plurality of piezoelectric elements of the base actuator unit. The pulse signals for double discharging utilize a predetermined pulse width (for example, W 1 ′) as the pulse width for the first pulse signal. A fixed value (for example, WS) is utilized as the pulse width for the second pulse signal. The time between the first pulse signal and the second pulse signal utilizes the pulse width (for example, W 1 ′) of the first pulse signal. The average value of the discharge speed of the ink droplets discharged from the plurality of nozzles is calculated. Here, the average value of the discharge speed of the ink droplets is calculated after the two ink droplets have merged. 
     (S 4 -2-2) The process of (S 4 -2-1) is executed with varying pulse widths for the first pulse signal. The average value of the ink droplet discharge speed for each of the pulse widths is calculated. 
     (S 4 -2-3) The results obtained in (S 4 -2-1) and (S 4 -2-2) are plotted in a graph in which pulse width is on the horizontal axis and discharge speed is on the vertical axis. Then a curved line is drawn passing through the points that have been plotted. When the curved line is drawn, the pulse width WD 1  in which the maximum discharge speed can be obtained is specified. 
     (S 4 -2-4) The process of (S 4 -2-1) is executed utilizing the fixed value WD 1  (the pulse width that was specified in (S 4 -2-3)) as the pulse width of the first pulse signal, and utilizing a predetermined value as the pulse width of the second pulse signal. 
     (S 4 -2-5) The process of (S 4 -2-4) is executed with varying pulse widths for the second pulse signal. The average value of the ink droplet discharge speed for each of the pulse widths is calculated. 
     (S 4 -2-6) The results obtained in (S 4 -2-4) and (S 4 -2-5) are plotted in a graph in which pulse width is on the horizontal axis and discharge speed is on the vertical axis. Then a curved line is drawn passing through the points that have been plotted. When the curved line is drawn, the pulse width WD 2  in which the maximum discharge speed can be obtained is specified. 
     (S 4 -2-7) TD 0  is zero. TD 1  is a value that is half of WD 1  obtained in (S 4 -2-3). TD 2  is a value where WD 1  has been added to TD 1 . The time between TD 1  and TD 2  is the pulse width WD 0  (see  FIG. 11(A) ). TD 3  is a value where the pulse width WD 1  has been added to TD 2 . TD 4  is a value obtained by adding TD 3  and WD 2  that was obtained in (S 4 -2-6). The time between TD 3  and TD 4  is the pulse width WD 2  (see  FIG. 11(A) ). A predetermined fixed value (a value identical with TS 3 ) is utilized as TD 5 . 
     (S 4 -3) The base pulse signals for triple discharging are specified. That is, TT 0  to TT 7  of  FIG. 8  are specified. This process is executed as follows. 
     (S 4 -3-1) Pulse signals for triple discharging are applied to the plurality of piezoelectric elements of the base actuator unit. The pulse signals for triple discharging utilize a predetermined pulse width (for example, W 1 ″) as the pulse width for a first pulse signal. A fixed value (for example, WS) is utilized as the pulse width for a second pulse signal. The time between the first pulse signal and the second pulse signal utilizes the pulse width (for example, W 1 ″) of the first pulse signal. A fixed value (for example, WS) is utilized as the pulse width for a third pulse signal. The time between the second pulse signal and the third pulse signal is utilized as the pulse width (for example, WS) of the second pulse signal. The average value of the discharge speed of the ink droplets discharged from the plurality of nozzles is calculated. Here, the average discharge speed of the ink droplets is calculated after the three ink droplets have merged. 
     (S 4 -3-2) The process of (S 4 -3-1) is executed with varying pulse widths for the first pulse signal. The average value of the ink droplet discharge speed for each of the pulse widths is calculated. 
     (S 4 -3-3) The results obtained in (S 4 -3-1) and (S 4 -3-2) are plotted in a graph in which pulse width is on the horizontal axis and discharge speed is on the vertical axis. Then a curved line is drawn passing through the points that have been plotted. When the curved line is drawn, the pulse width WT 1  in which the maximum discharge speed can be obtained is specified. 
     (S 4 -3-4) The process of (S 4 -3-1) is executed utilizing the fixed value WT 1  (the pulse width that was specified in (S 4 -3-3)) as the pulse width of the first pulse signal, utilizing a predetermined value as the pulse width of the second pulse signal, and utilizing the fixed value (for example, WS) as the pulse width of the third pulse signal. 
     (S 4 -3-5) The process of (S 4 -3-4) is executed with varying pulse widths for the second pulse signal. The average value of the ink droplet discharge speed for each of the pulse widths is calculated. 
     (S 4 -3-6) The results obtained in (S 4 -3-4) and (S 4 -3-5) are plotted in a graph in which pulse width is on the horizontal axis and discharge speed is on the vertical axis. Then a curved line is drawn passing through the points that have been plotted. When the curved line is drawn, the pulse width WT 2  in which the maximum discharge speed can be obtained is specified. 
     (S 4 -3-7) The process of (S 4 -3-1) is executed utilizing the fixed value WT 1  (the pulse width that was specified in (S 4 -3-3)) as the pulse width of the first pulse signal, utilizing the fixed value WT 2  (the pulse width that was specified in (S 4 -3-6)) as the pulse width of the second pulse signal, and utilizing a predetermined value as the pulse width of the third pulse signal. 
     (S 4 -3-8) The process of (S 4 -3-7) is executed with varying pulse widths for the third pulse signal. The average value of the ink droplet discharge speed for each of the pulse widths is calculated. 
     (S 4 -3-9) The results obtained in (S 4 -3-7) and (S 4 -3-8) are plotted in a graph in which pulse width is on the horizontal axis and discharge speed is on the vertical axis. Then a curved line is drawn passing through the points that have been plotted. When the curved line is drawn, the pulse width WT 3  in which the maximum discharge speed can be obtained is specified. 
     (S 4 -3-10) TT 0  is zero. TT 1  is a value that is half of WT 1  obtained in (S 4 -3-3). TT 2  is a value where WT 1  has been added to TT 1 . The time between TT 1  and TT 2  is the pulse width WT 1  (see  FIG. 12  (A)). TT 3  is a value where the pulse width WT 1  has been added to TT 2 . TT 4  is a value obtained by adding TT 3  and WT 2  that was obtained in (S 4 -3-6). The time between TT 3  and TT 4  is the pulse width WT 2  (see  FIG. 12  (A)). TT 5  is a value where WT 2  has been added to TT 4 . TT 6  is a value where the pulse width WT 3  obtained in (S 4 -3-9) has been added to TT 5 . The time between TT 5  and TT 6  is the pulse width WT 3  (see  FIG. 12  (A)). A predetermined fixed value (a value identical with TS 3  and TD 5 ) is utilized as TT 7 . 
     The ink jet printer is prepared after executing the processes of S 4  of  FIG. 13 . This ink jet printer contains programs for creating the pulse signals by multiplying the base pulse signals obtained in the processes of S 4  by the coefficients. For example, as described above, the ink jet printer  1  that has the four ink jet heads  2   a  to  2   d  is manufactured. The specific coefficients are not stored in the coefficient storage  204  of  FIG. 7  at this step. To deal with this, the processes of S 6  of  FIG. 13  are executed. In S 6 , the coefficients (α 1 to α 16 ) of the printer  1  are determined. This process is executed as follows. 
     (S 6 -1) The coefficient of one actuator unit is determined. Here, the determination of the coefficient α 1  of the actuator unit  21   a  of the ink jet head  2   a  will be described as an example. 
     (S 6 -1-1) α 1  is input as a predetermined value. α 1  can be input utilizing, for example, the operation panel  250  (see  FIG. 1 , etc). Then, a pulse signal (a pulse signal for single discharging) is applied to the piezoelectric elements  20  of the actuator unit  21   a  of the ink jet head  2   a . The pulse signal that is applied has a pulse width of α 1 ×WS. The discharge speed of the ink droplets discharged from the nozzles is measured. The average value of the measured discharge speed is calculated. 
     (S 6 -1-2) The process of (S 6 -1-1) is executed with varying values for the coefficient α 1 . The average value of the ink droplet discharge speed for each of the coefficients α 1  is calculated. 
     (S 6 -1-3) The results obtained in (S 6 -1-1) and (S 6 -1-2) are plotted in a graph in which pulse width is on the horizontal axis and discharge speed is on the vertical axis. Then a curved line is drawn passing through the points that have been plotted. The curved line R 1  in  FIG. 14  is an example of this curved line. When the curved line is drawn, the pulse width AL 1  in which the maximum discharge speed can be obtained is specified. 
     (S 6 -1-4) The pulse width AL 1  obtained in (S 6 -1-3) is divided by the base pulse width WS of the pulse signal for single discharging, thus obtaining α 1 . 
     (S 6 -2) The same process (S 6 -1) is executed for the other actuator units. For example, the process is executed for the actuator unit  21   b  of the ink jet head  2   a . In this case, the graph of R 2  of  FIG. 14  is obtained. The pulse width AL 2  specified from the graph R 2  is divided by the base pulse width WS, thus obtaining α 2 . 
     As another example, the process is executed for the actuator unit  21   c  of the ink jet head  2   a . In this case, the graph of R 3  of  FIG. 14  is obtained. The pulse width AL 3  specified from the graph R 3  is divided by the base pulse width WS, thus obtaining α 3 . 
     As another example, the process is executed for the actuator unit  21   d  of the ink jet head  2   a . In this case, the graph of R 4  of  FIG. 14  is obtained. The pulse width AL 4  specified from the graph R 4  is divided by the base pulse width WS, thus obtaining α 4 . 
     The same process is executed for the other ink jet heads  2   b  to  2   d , thereby obtaining α 5  to α 16 . 
     When the process of S 6  of  FIG. 13  has been completed, the process proceeds to S 8 . In S 8 , α 1  to α 16  that were calculated in S 6  are input to the inkjet printer  1 . α 1  to α 16  can be input utilizing the operation panel  250  (see  FIG. 1 , etc.). The coefficients that have been input are stored in the coefficient storage  204  of  FIG. 7 . The ink jet printer  1  is thus completed. 
     According to the present embodiment, the pulse width in which the maximum discharge speed of the ink droplets can be obtained during single discharging is obtained in (S 6 -1-3). Then this pulse width is divided by the base pulse width WS, thereby obtaining the coefficient. The printer  1  multiplies the coefficient that has been obtained by the base pulse width WS, thereby creating the pulse signal for single discharging. That is, the pulse width in which the maximum discharge speed of the ink droplets can be obtained is utilized for single discharging. When the pulse width has been determined utilizing the coefficient that has been obtained, satisfactory printing results can be achieved. 
     Further, the coefficient that has been obtained is also utilized for creating the pulse signals for double discharging and the pulse signals for triple discharging. That is, when the coefficient that was determined based on single discharging is multiplied by the base pulse signals for double discharging, the pulse signals for double discharging are created. Further, when the coefficient that was determined based on single discharging is multiplied by the base pulse signals for triple discharging, the pulse signals for triple discharging are created. The present inventors realized from their research that, if satisfactory printing results can be achieved by executing single discharging utilizing the base pulse width and the coefficient that has been obtained, satisfactory printing results can also be achieved by executing double discharging and triple discharging utilizing that coefficient. 
     In the present embodiment, it is possible to create the pulse signal for single discharging, the pulse signals for double discharging, and the pulse signals for triple discharging merely by inputting one coefficient for one actuator unit. A plurality of pulse signals that allow satisfactory printing results to be achieved can be created merely by inputting a comparatively small amount of data. 
     SECOND EMBODIMENT 
     Only parts differing from the first embodiment will be described. In the present embodiment, the process of S 6  of  FIG. 13  differs from the first embodiment. In particular, the processes of (S 6 -1-3) and (S 6 -1-4) differ from the first embodiment. In (S 6 -1-3), if for example the curved line R 1  of  FIG. 15  is obtained, the pulse width AL 1  in which the maximum discharge speed can be obtained is specified. In the present embodiment, the range of the discharge speed is set to be F 1  to F 4 . Then it is specified whether the pulse width AL 1  that has been specified is included in any of these ranges (F 1  in this example). A representative value AL 1 ′of that range F 1  is specified. The representative value AL 1 ′is an intermediate value of the range F 1 . 
     If the pulse width in which the maximum discharge speed can be obtained is included in the range F 2  (in the case of the graph R 2  of  FIG. 15 ), a representative value AL 2 ′ of the range F 2  is specified. The representative value AL 2 ′ is an intermediate value of the range F 2 . If the pulse width in which the maximum discharge speed can be obtained is included in the range F 3  (in the case of the graph R 3  of  FIG. 15 ), a representative value AL 3 ′ of the range F 3  is specified. The representative value AL 3 ′ is an intermediate value of the range F 3 . If the pulse width in which the maximum discharge speed can be obtained is included in the range F 4  (in the case of the graph R 4  of  FIG. 15 ), a representative value AL 4 ′ of the range F 4  is specified. The representative value AL 4 ′ is an intermediate value of the range F 4 . 
     In (S 6 - 1 - 4 ), the representative value (for example, AL 1 ′) obtained in (S 6 - 1 - 3 ) is divided by the base pulse width WS for single discharging. The coefficient (for example, α 1 ) can thus be obtained. 
     The coefficients for the other actuator units can be obtained by executing the same process. 
     THIRD EMBODIMENT 
     Only parts differing from the first embodiment will be described. In the present embodiment, the coefficient storage  204  of  FIG. 7  stores coefficients for each of the piezoelectric elements. For example, if one actuator unit  21  has 1000 piezoelectric elements  20 , the printer requires 16000 coefficients. 
     The print signal creating portion  206  determines the pulse signals to be applied to each of the piezoelectric elements  20  by multiplying the base pulse signal by the coefficient of that piezoelectric element  20 . For example, if the coefficient of a piezoelectric element  20 A is αA, the pulse signal of the piezoelectric element  20 A is determined by multiplying the base pulse signal by αA. Further, if the coefficient of a piezoelectric element  20 B is αB, the pulse signal of the piezoelectric element  20 B is determined by multiplying the base pulse signal by αB. 
     In the case of the present embodiment, the process of S 6  of  FIG. 13  differs from the first embodiment. In S 6 , the coefficient of each of the piezoelectric elements is determined. 
     (S 6 -1′) Here, the case in which the coefficient of the piezoelectric element  20 A is determined will be given as an example. 
     (S 6 -1′-1) A predetermined value is input as the coefficient αA of the piezoelectric element  20 A. A pulse signal (a pulse signal for single discharging) is applied to the piezoelectric element  20 A. The pulse signal that is applied has a pulse width of αA×WS in which αA is multiplied by the base pulse width WS. The discharge speed of the ink droplet is measured. 
     (S 6 -1′-2) The process of (S 6 -1′-1) is executed with varying values for the coefficient αA. The discharge speed of the ink droplets for each of the coefficients αA is calculated. 
     (S 6 -1′-3) The results obtained in (S 6 -1′-1) and (S 6 -1′-2) are plotted in a graph in which pulse width is on the horizontal axis and discharge speed is on the vertical axis. Then a curved line is drawn passing through the points that have been plotted. When the curved line is drawn, the pulse width ALA in which the maximum discharge speed can be obtained is specified. 
     (S 6 -1′-4) The pulse width ALA obtained in (S 6 -1′-3) is divided by the base pulse width WS of the pulse signal for single discharging, thus obtaining αA. 
     (S 6 -2′) The same process of (S 6 -1′) is executed for the other piezoelectric elements  20 . The coefficient of each of the piezoelectric elements  20  can thus be obtained. 
     The coefficients that have been obtained are input to the printer  1  in S 8  of  FIG. 13 . 
     FOURTH EMBODIMENT 
     Only parts differing from the first embodiment will be described. In the present embodiment, the coefficient storage  204  of  FIG. 7  stores coefficients of each of the ink jet heads. That is, a coefficient of the ink jet head  2   a , a coefficient of the ink jet head  2   b , a coefficient of the ink jet head  2   c , and a coefficient of the ink jet head  2   d  are stored. Only four coefficients are stored in the coefficient storage  204 . 
     The print signal creating portion  206  determines the pulse signals to be applied to each of the piezoelectric elements  20  by multiplying the base pulse signal by the coefficient of the ink jet head (for example,  2   a ) that has the piezoelectric elements  20 . 
     In the case of the present embodiment, the process of S 6  of  FIG. 13  differs from the first embodiment. In S 6 , the coefficients of the four ink jet heads  2   a  to  2   d  are determined. 
     (S 6 -1″) The coefficient of one ink jet head is determined. Here, the case in which the coefficient αA of the ink jet head  2   a  is determined will be given as an example. 
     (S 6 -1″-1) A predetermined value is input as the coefficient αA. A pulse signal (a pulse signal for single discharging) is applied to some of the piezoelectric elements  20 A included in the ink jet head  2   a . It is preferred that the piezoelectric elements  20  to which the pulse signal is applied are selected from each of the actuator units  21   a  to  21   d . For example, one piezoelectric element  20  can be chosen from each of the actuator units  21   a  to  21   d . The pulse signal that is applied has a pulse width of αA×WS in which αA is multiplied by the base pulse width WS. The discharge speed of the ink droplet discharged from each nozzle is measured. The average value of the measured discharge speed is calculated. 
     (S 6 -1″-2) The process of (S 6 -1″-1) is executed with varying values for the coefficient αA. The discharge speed of the ink droplets for each of the coefficients αA is calculated. 
     (S 6 -1″-3) The results obtained in (S 6 -1″-1) and (S 6 -1″-2) are plotted in a graph in which pulse width is on the horizontal axis and discharge speed is on the vertical axis. Then a curved line is drawn passing through the points that have been plotted. When the curved line is drawn, the pulse width ALA in which the maximum discharge speed can be obtained is specified. 
     (S 6 -1″-4) The pulse width ALA obtained in (S 6 -1″-3) is divided by the base pulse width WS of the pulse signal for single discharging, thus obtaining αA. 
     (S 6 -2″) The same process of (S 6 -1″) is executed for the other ink jet heads  2   b , etc. The coefficients of the ink jet heads  2   a  to  2   d  can thus be obtained. 
     The coefficients that have been obtained are input to the printer  1  in S 8  of  FIG. 13 . 
     Some representative modifications to the aforementioned embodiments are listed here. 
     (1) The aforementioned embodiments can be applied to a serial type printer in which the ink jet heads move with a printer main body. 
     (2) The operation panel  250  (see  FIG. 7 ) need not be utilized to input the coefficients. For example, the coefficients may be input utilizing the PC  252 . The coefficients input utilizing the PC  252  are input to the inputting portion  210  of  FIG. 7 . The coefficients that have been input are stored in the coefficient storage  204 . 
     (3) The process of S 8  of  FIG. 13  may be executed by the manufacturer of the printer  1 , or by the user of the printer  1 . If executed by the user of the printer  1 , the manufacturer of the printer  1  executes a process of informing the user of the results (i.e. the coefficients) of the process of S 6 . 
     (4) In the base pulse signal for double discharging, the pulse width WD 1  of the first pulse signal and the pulse width WD 2  of the second pulse signal may be identical. 
     In this case, (S 4 -2) of the first embodiment may be modified as follows. 
     (S 4 -2-1) Pulse signals for double discharging are applied to the plurality of piezoelectric elements of the base actuator unit. The pulse signals for double discharging utilize a predetermined pulse width (for example, W 1 ′) as the pulse width for the first pulse signal. The pulse width for the second pulse signal is the same as the pulse width (for example, W 1 ′) for the first pulse signal. The time between the first pulse signal and the second pulse signal utilizes the pulse width (for example, W 1 ′) of the first pulse signal. The average value of the discharge speed of the ink droplets discharged from the plurality of nozzles is calculated. 
     (S 4 -2-2) The process of (S 4 -2-1) is executed with varying pulse widths. The pulse width for the first pulse signal and the pulse width for the second pulse signal are the same. The average value of the discharge speed of the ink droplets for each of the pulse widths is calculated. 
     (S 4 -2-3) The results obtained in (S 4 -2-1) and (S 4 -2-2) are plotted in a graph in which pulse width is on the horizontal axis and discharge speed is on the vertical axis. Then a curved line is drawn passing through the points that have been plotted. When the curved line is drawn, the pulse width WD 1  in which the maximum discharge speed can be obtained is specified. The same value as in the pulse width WD 1  is utilized in the pulse width WD 2 . The processes of (S 4 -2-4) to (S 4 -2-6) are not executed. The process of (S 4 -2-7) is the same as in the first embodiment. 
     (5) In the base pulse signal for triple discharging, the pulse width WT 1  of the first pulse signal, the pulse width WT 2  of the second pulse signal, and the pulse width WT 3  of the third pulse signal may be identical. 
     In this case, (S 4 -3) of the first embodiment can be modified as follows. 
     (S 4 -3-1) Pulse signals for triple discharging are applied to the plurality of piezoelectric elements of the base actuator unit. The pulse signals for triple discharging utilize a predetermined pulse width (for example, W 1 ″) as the pulse width for the first pulse signal. The pulse widths for the second pulse signal and the third pulse signal use the same value as the pulse width (for example, W 1 ″) for the first pulse signal. The time between the first pulse signal and the second pulse signal utilizes the pulse width (for example, W 1 ″) of the first pulse signal. The time between the second pulse signal and the third pulse signal is utilized as the pulse width of the second pulse signal (i.e. the pulse width of the first pulse signal). The average value of the discharge speed of the ink droplets discharged from the plurality of nozzles is calculated. 
     (S 4 -3-2) The process of (S 4 -3-1) is executed with varying pulse widths. The pulse widths for the first pulse signal, the second pulse signal and the third pulse signal are the same. The average value of the discharge speed of the ink droplets for each of the pulse widths is calculated. 
     (S 4 -3-3) The results obtained in (S 4 -3-1) and (S 4 -3-2) are plotted in a graph in which pulse width is on the horizontal axis and discharge speed is on the vertical axis. Then a curved line is drawn passing through the points that have been plotted. When the curved line is drawn, the pulse width WT 1  in which the maximum discharge speed can be obtained is specified. The same value as in the pulse width WT 1  is utilized in the pulse width WT 2  and the pulse width WT 3 . The processes of (S 4 -3-4) to (S 4 -3-9) are not executed. The process of (S 4 -3-10) is the same as in the first embodiment. 
     (6) At least two of the six base pulse widths WS, WD 1 , WD 2 , WT 1 , WT 2 , WT 3  of the present embodiments may be identical pulse widths. For example, WS, WD 1 , and WT 1  may be identical pulse widths. 
     (7) In the aforementioned embodiments, the print signal creating portion  206  (see  FIG. 7 ) multiplies the base pulse signals and the coefficients when the print signals are created. However, the base pulse signals and the coefficients may be multiplied when the coefficients are input. In this case, various kinds of pulse signals can be obtained before the print signals are created. If this is done, calculation is not required at the time of printing.