Patent Publication Number: US-8109590-B2

Title: Liquid ejecting apparatus and method of setting signal for micro vibration

Description:
CROSS-REFERENCE TO A RELATED APPLICATION 
     This application is a Division of U.S. patent application Ser. No. 12/403,482 filed Mar. 13, 2009, which claimed priority to Japanese Patent Application Number 2008-066415 filed Mar. 14, 2008. The entire disclosures of these applications are expressly incorporated herein by reference. 
    
    
     BACKGROUND 
     Technical Field 
     The present invention relates to a liquid ejecting apparatus and a method of setting a signal for micro vibration. 
     As a liquid ejecting apparatus ejecting a liquid, an ink jet type printer is known in which ink droplets are ejected from nozzles. Such an ink jet type printer includes a printer in which ink is prevented from being thickened near the nozzles. In this printer, for example, in order to cause micro vibration of a meniscus (a free surface of ink to be exposed from the nozzles), a pulse for micro vibration (potential change pattern) is applied to a piezoelectric element. If the pulse for micro vibration is applied, weak pressure vibration is applied to ink in a pressure chamber to such an extent that ink is not ejected. 
     JP-A-2000-117993 is an example of the related art. 
     With respect to the pressure vibration, amplitude or attenuation time is an important factor. For example, if the amplitude is extremely large, ink droplets may be ejected with irregular timing. If the amplitude is extremely small, thickening is insufficiently suppressed. In addition, if the attenuation time is extremely long, the amount of ink droplets to be ejected from the nozzles may be influenced. If the attenuation time is extremely short, ink may be thickened after attenuation. 
     For this reason, it is necessary to optimize the amplitude or attenuation time of the pressure vibration to be applied to ink. 
     SUMMARY 
     An advantage of some aspects of the invention is that it optimizes a signal for micro vibration. 
     According to an aspect of the invention, a liquid ejecting apparatus includes liquid chambers in which a liquid is filled, nozzles communicating with the liquid chambers, a signal generator generating a signal of potential change, and elements operating in accordance with the potential of the signal to be applied to cause a change in pressure of the liquid filled in the liquid chambers. The signal generator generates a signal for micro vibration which causes micro vibration of a free surface of the liquid to be exposed from the nozzles to such an extent that the liquid is not ejected. The signal for micro vibration has a first potential change portion at which a potential changes from a first potential to a medium potential defined between the first potential and a second potential, a constant potential portion which is generated after the first potential change portion and at which the potential is maintained constant at the medium potential, and a second potential change portion which is generated after the constant potential portion and at which the potential changes from the medium potential to the second potential. 
     Other features of the invention will be apparent from the specification and the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIG. 1  is a diagram schematically showing the configuration of an ink jet type printer according to a first embodiment of the invention. 
         FIG. 2  is a partially enlarged sectional view specifically showing the internal configuration of a line head in  FIG. 1 . 
         FIG. 3  is a sectional view schematically showing an example where ink droplets are ejected from nozzles in the line head of  FIG. 2 . 
         FIG. 4  is a diagram illustrating a driving signal which is generated by a signal generator in  FIG. 1 . 
         FIGS. 5A and 5B  are diagrams illustrating dot forming data. Specifically,  FIG. 5A  is a diagram illustrating the relationship between the size of a dot to be formed and dot forming data, and  FIG. 5B  is a diagram illustrating the relationship between dot forming data and a pulse to be applied. 
         FIG. 6  is an enlarged view of a pulse for micro vibration shown in  FIG. 4 . 
         FIGS. 7A to 7C  are diagrams showing the state of a meniscus before and when the pulse for micro vibration shown in  FIG. 6  is applied to a piezoelectric element. Specifically,  FIG. 7A  is a diagram showing the state of a meniscus before the pulse for micro vibration is applied,  FIG. 7B  is a diagram showing an example of a state when a meniscus is pulled in toward a pressure chamber by application of the pulse for micro vibration, and  FIG. 7C  is a diagram showing an example of a state where a meniscus is pushed out toward a side opposite a pressure chamber by application of the pulse for micro vibration. 
         FIGS. 8A and 8B  are diagrams illustrating pressure vibration to be applied to ink in a pressure chamber when the pulse for micro vibration shown in  FIG. 6  is applied to a piezoelectric element. Specifically,  FIG. 8A  shows pressure vibration to be applied to ink in a pressure chamber due to a first charging portion and pressure vibration to be applied to ink in a pressure chamber due to a second charging portion, and  FIG. 8B  shows a composite waveform of two kinds of pressure vibration shown in  FIG. 8A . 
         FIG. 9  is a diagram illustrating a pulse for micro vibration according to a second embodiment of the invention. 
         FIG. 10  is a diagram illustrating pressure vibration to be applied to ink in a pressure chamber when the pulse for micro vibration shown in  FIG. 9  is applied to a piezoelectric element. 
         FIG. 11  is a diagram illustrating another pulse for micro vibration different from those shown in  FIGS. 6 and 9 . 
         FIGS. 12A and 12B  are diagrams illustrating another pulse for micro vibration different from those shown in  FIGS. 6 and 9 . Specifically,  FIG. 12A  shows a pulse for micro vibration in which a potential change pattern of a charging portion is linear, and  FIG. 12B  shows a pulse for micro vibration in which a potential change pattern of a charging portion is curved. 
         FIG. 13  is a diagram illustrating another driving signal different from that shown in  FIG. 4 . 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     At least the following will be apparent from the specification and the accompanying drawings. 
     A liquid ejecting apparatus includes liquid chambers in which a liquid is filled, nozzles communicating with the liquid chambers, a signal generator generating a signal of potential change, and elements operating in accordance with the potential of the signal to be applied to cause a change in pressure of the liquid filled in the liquid chambers. The signal generator generates a signal for micro vibration which causes micro vibration of a free surface of the liquid to be exposed from the nozzles to such an extent that the liquid is not ejected. The signal for micro vibration has a first potential change portion at which a potential changes from a first potential to a medium potential defined between the first potential and a second potential, a constant potential portion which is generated after the first potential change portion and at which the potential is maintained constant at the medium potential, and a second potential change portion which is generated after the constant potential portion and at which the potential changes from the medium potential to the second potential. 
     With this liquid ejecting apparatus, an interval between the first potential change portion and the second potential change portion, and the medium potential can be set. Therefore, the amplitude or attenuation time of pressure vibration to be applied to the liquid filled in the liquid chambers can be adjusted. As a result, the amplitude or attenuation time of pressure vibration to be applied to the liquid can be optimized. 
     A generation start timing of the second potential change portion may be defined within a range represented by Expression (1) starting with a generation start timing of the first potential change portion.
 
nTc+0.5Tc±0.25Tc  (1)
 
     Here, n is an integer of 0 or more, and Tc is a cycle intrinsic to the pressure vibration to be applied to the liquid. 
     With this configuration, when the second potential change portion starts to be applied, the pressure vibration applied to the liquid due to the first potential change portion can be prevented from being extremely excited. 
     A difference between the medium potential and the second potential may be larger than a difference between the medium potential and the first potential. With this configuration, the attenuation time of pressure vibration can be appropriately adjusted. 
     A generation start timing of the second potential change portion may be defined within a range represented by Expression (2) starting with a generation start timing of the first potential change portion.
 
mTc±0.25Tc  (2)
 
     Here, m is an integer of 0 or more, and Tc is a cycle intrinsic to the pressure vibration to be applied to the liquid. 
     With this configuration, when the second potential change portion starts to be applied, even if the pressure vibration applied to the liquid due to the first potential change portion is attenuated, the pressure vibration can be efficiently excited. 
     A difference between the medium potential and the second potential may be smaller than a difference between the medium potential and the first potential. With this configuration, the amplitude of the pressure vibration applied to the liquid due to the first potential change portion and the second potential change portion can be optimized. 
     The liquid ejecting apparatus may further include a pulse generator for defining a generation timing of a liquid ejection signal so as to eject the liquid from the nozzles. The signal may include a first signal having the signal for micro vibration, and a second signal having no signal for micro vibration and having the liquid ejection signal. The pulse may be generated within a generation period of the constant potential portion in the signal for micro vibration. 
     With this configuration, while the signal for micro vibration is being applied, an influence (noise) of a pulse due to switching of the second signal can be substantially eliminated. 
     Another liquid ejecting apparatus includes liquid chambers in which a liquid is filled, nozzles communicating with the liquid chambers, a signal generator generating a signal of potential change, and elements operating in accordance with the potential of the signal to be applied to cause a change in pressure of the liquid filled in the liquid chambers. The signal generator generates a signal for micro vibration which causes micro vibration of a free surface of the liquid to be exposed from the nozzles to such an extent that the liquid is not ejected. The signal for micro vibration has a first potential change portion at which a potential changes from a first potential to a medium potential defined between the first potential and a second potential, and a second potential change portion which is generated after the first potential change portion and at which the potential changes from the medium potential to the second potential. The potential change amount per unit time of the second potential change portion is different from that of the first potential change portion. 
     With this liquid ejecting apparatus, the pressure vibration applied to the liquid due to the first potential change portion can be adjusted by a change in pressure due to the second potential change portion. Therefore, the amplitude or attenuation time of the pressure vibration can be optimized. 
     There is provided a method of setting a signal for micro vibration, which is applied to elements causing a change in pressure of a liquid in liquid chambers to cause micro vibration of a free surface of the liquid to be exposed from nozzles communicating with the liquid chambers to such an extent that the liquid is not ejected. The method includes setting potential information required for changing a potential from a first potential to a medium potential defined between the first potential and a second potential, setting potential information required for, after the potential has changed from the first potential to the medium potential, maintaining the potential constant at the medium potential, and setting potential information required for, after the potential has been maintained at the medium potential, changing the potential from the medium potential to the second potential. 
     With this method of setting a signal for micro vibration, an interval between the first potential change portion and the second potential change portion, and the medium potential can be set to have a desired magnitude. With this configuration, the amplitude or attenuation time of pressure vibration to be applied to the liquid filled in the liquid chambers can be adjusted. As a result, the amplitude or attenuation time of pressure vibration to be applied to the liquid can be optimized. 
     Embodiments of the invention will now be described with reference to the drawings. 
     First Embodiment 
       FIG. 1  is a diagram schematically showing the configuration of an ink jet type printer according to a first embodiment of the invention.  FIG. 2  is a partially enlarged sectional view specifically showing the internal configuration of a line head in  FIG. 1 .  FIG. 3  is a sectional view schematically showing an example where ink droplets are ejected from nozzles in the line head of  FIG. 2 .  FIG. 4  is a diagram illustrating a driving signal which is generated by a signal generator in  FIG. 1 . 
     Printer 
     A printer  1  shown in  FIG. 1  has a controller  10 , a signal generator  20 , and a head unit  50 . The printer  1  prints an image on a sheet, which is an example of a printing medium, while transporting the sheet in a predetermined direction. During printing, in the printer  1 , ink is ejected from nozzles provided in the head unit  50  in the shape of droplets. Ink is a kind of liquid. Therefore, the printer  1  is a kind of liquid ejecting apparatus. The printer  1  defines the size of a dot to be formed on the sheet as one of three kinds (S, M, and L) during printing, and adjusts the amount of an ink droplet to be ejected (for example, volume) in accordance with the defined dot size. In this way, the amount of the ink droplet is adjusted, thereby increasing image quality of a printed matter. 
     Head Unit  50   
     As shown in  FIG. 1 , the head unit  50  has a line head  60 , a head driver  72 , and a control signal generating circuit  73 . 
     Line Head  60   
     As shown in  FIGS. 2 and 3 , the line head  60  includes a plurality of piezoelectric elements  61  and an ink flow channel  62 . The ink flow channel  62  has an upstream portion extending to a common ink chamber  62   a  and individual portions from the common ink chamber  62   a  to the nozzles (holes). The individual portions are provided to correspond to the number of piezoelectric elements  61 . When the printer  1  is used, the ink flow channel  62  is filled with ink. Pressure chambers  63  are provided in the ink flow channel  62  (that is, in the individual portions). The pressure chambers  63  correspond to liquid chambers in which a liquid is filled, and are partially partitioned by vibrating plates  64 . 
     If driving signals COM 1  and COM 2  are applied, the piezoelectric elements  61  are charged or discharged by a change in potential of each driving signal. The piezoelectric elements  61  are deformed when being charged or discharged. In this embodiment, each piezoelectric element  61  contracts in a longitudinal direction when being charged, and expands in the longitudinal direction when being discharged. As the piezoelectric element  61  is deformed, the vibrating plate  64  is deformed, and the volume of the pressure chamber  63  is changed. Accordingly, the pressure of ink in the pressure chamber  63  is changed. For this reason, the piezoelectric element  61  is an example of an element operating in accordance with the potential of a signal to be applied, to thereby cause a change in pressure of the liquid filled in the corresponding liquid chamber. The line head  60  is provided with nozzles corresponding to the number of piezoelectric elements  61 . The nozzles eject ink droplets and communicate with the pressure chambers  63 . 
     The pressure chambers  63  are arranged in an arrangement direction of the nozzles. Two adjacent pressure chambers  63  and  63  are provided with a thin partition wall interposed therebetween. Between the pressure chamber  63  and the piezoelectric element  61 , the vibrating plate  64  is provided. The vibrating plate  64  functions as a movable portion (also called a diaphragm) in the pressure chamber  63 . That is, the vibrating plate  64  has a thick portion  64   a  and a thin portion  64   b . The thick portion  64   a  is attached to a tip surface of the piezoelectric element  61 , and the thin portion  64   b  is formed of an elastic material having high elasticity, such as synthetic resin. 
     If the piezoelectric element  61  is deformed, that is, if the piezoelectric element  61  expands or contracts in the longitudinal direction, the thin portion  64   b  also expands or contracts. Accordingly, the thick portion  64   a  is pressed toward the pressure chamber  63  or is pulled toward a side opposite the pressure chamber  63 . If the thick portion  64   a  is pressed toward the pressure chamber  63 , the volume of the pressure chamber  63  decreases, and the pressure of ink in the pressure chamber  63  increases. To the contrary, if the thick portion  64   a  is pulled toward the side opposite the pressure chamber  63 , the volume of the pressure chamber  63  increases, and the pressure of ink in the pressure chamber  63  decreases. Therefore, by control of the pressure of ink in the pressure chamber  63 , ink can be ejected from the nozzle, and micro vibration of a meniscus can be generated (described below). 
     Head Driver  72   
     The head driver  72  has a plurality of switches. In the printer  1 , each pair of switches has two switches  72   a  and  72   b . Pairs of switches each having the switches  72   a  and  72   b  are provided to correspond to the number of piezoelectric elements  61 . The switches constituting each pair of switches are provided to correspond to the number of driving signals. If the switches  72   a  and  72   b  are put in a conduction state, a corresponding driving signal is applied to the piezoelectric element  61 . 
     Control Signal Generating Circuit  73   
     The control signal generating circuit  73  is, for example, a logic circuit which generates a control signal in accordance with dot forming data and timing signals (described below) and inputs the generated control signal to the head driver  72 . The control signal is a signal for switching the switches  72   a  and  72   b  between a conduction state and a non-conduction state. With the control signal, the operation of the switch  72   a  or the switch  72   b  is controlled. 
     Signal Generator  20   
     As shown in  FIG. 1 , the signal generator  20  has a first driving signal generator  21  generating a first driving signal COM 1  and a second driving signal generator  22  generating a second driving signal COM 2 . The driving signals COM 1  and COM 2  are repeatedly generated for each repetition cycle T shown in  FIG. 4 . 
     Next, the driving signals to be generated by the signal generator  20  will be described. 
     As shown in  FIG. 4 , the driving signal COM 1  includes a large dot pulse L and a pulse N for micro vibration. The large dot pulse L is generated during a generation period TL. The pulse N for micro vibration is generated during a generation period TN. In addition, the driving signal COM 2  includes a medium dot pulse M and a small dot pulse S. The medium dot pulse M is generated during a generation period TM. The small dot pulse S is generated during a generation period TS. 
     The repetition cycle T of the driving signal COM 1  is divided into a period TL′ including the generation period TL of the large dot pulse L and a period TN′ including the generation period TN of the pulse N for micro vibration by pulses of a change signal CH 1 . The repetition cycle T of the driving signal COM 2  is divided into a period TM′ including the generation period TM of the medium dot pulse M and a period TS′ including the generation period TS of the small dot pulse S by pulses of a change signal CH 2 . The change signals CH 1  and CH 2  are examples of timing signals described below. 
     Each of the large dot pulse L, the medium dot pulse M, and the small dot pulse S is applied to the piezoelectric element  61  when a dot having a corresponding sizes (S, M, L) is formed. In other words, each of the large dot pulse L, the medium dot pulse M, and the small dot pulse S is used to eject an ink droplet of an amount suitable for the corresponding dot size. The pulses are portions of a liquid ejection signal. That is, the large dot pulse L is a portion of a liquid ejection signal for a large dot which is generated during the generation period TL′. The medium dot pulse M is a portion of a liquid ejection signal for a medium dot which is generated during the generation period TM&#39;. Similarly, the small dot pulse S is a portion of a liquid ejection signal for a small dot which is generated during the generation period TS′. Hereinafter, the three pulses are also collectively referred to as ejection pulses. 
     The pulse N for micro vibration is used to cause micro vibration of a meniscus when ink is not ejected. If the pulse N for micro vibration is applied to the piezoelectric element  61 , ink in the pressure chamber  63  undergoes a change in pressure to such an extent that ink is not ejected from the nozzle. The change in pressure causes micro vibration of the meniscus. Therefore, the pulse N for micro vibration is a non-ejection pulse to suppress ink ejection from the nozzles, and is a portion of the signal for micro vibration to cause micro vibration of the meniscus. That is, the pulse N for micro vibration is a portion of the signal for micro vibration which is generated during the generation period TN′. The pulse N for micro vibration and a micro vibration operation using the pulse N for micro vibration will be described below in detail. 
     As described above, the driving signal COM 1  includes the liquid ejection signal for a large dot and the signal for micro vibration, and the driving signal COM 2  includes the liquid ejection signal for a medium dot and the liquid ejection signal for a small dot. From this, it can be considered that the driving signal COM 1  is an example of a first signal having a signal for micro vibration, and the driving signal COM 2  is an example of a second signal having no signal for micro vibration and having a liquid ejection signal. 
     Controller  10   
     As shown in  FIG. 1 , the controller  10  has a CPU  11  controlling the individual sections of the printer  1 , a memory  12  serving as a storage medium, an interface (I/F)  13  disposed in the printer  1 , and an internal bus  15  connecting the CPU  11 , the memory  12 , and the I/F  13 . 
     The memory  12  stores programs and various kinds of data. The programs include a program (firmware) for control of the individual sections of the printer  1 . Data includes image data to be printed and waveform generation information. Two kinds of the waveform generation information are present. The waveform generation information is digital data in which potential information of each of the driving signals COM 1  and COM 2  is arranged in time series. 
     The CPU  11  reads out and executes a program stored in the memory  12  to control sheet transport, generation of the driving signals by the signal generator  20 , and ejection of ink droplets by the head unit  50 . 
     In order to control ejection of ink droplets, the CPU  11  generates dot forming data and timing signals, and inputs the generated dot forming data and timing signals to the control signal generating circuit  73 . Dot forming data is generated from image data to be printed. The timing signals collectively refer to a latch signal LAT and the change signals CH 1  and CH 2 , and include pulses defining timing for control such that a dot is formed in each unit area of a predetermined size or no dot is formed. The pulses of the latch signal LAT are generated in the same cycle as the repetition cycle T. The pulses of the change signal CH 1  are generated during the repetition cycle T. The pulses of the change signal CH 2  are also generated during the repetition cycle T. In this example, the pulses of the change signal CH 2  are generated with timing different from those of the change signal CH 1  (see  FIG. 4 ). The CPU  11 , that is, the controller  10  is a kind of pulse generator that generates a pulse for defining a generation timing of the liquid ejection signal to be used to eject the liquid from the nozzle. 
     In order to control generation of the driving signals by the signal generator  20 , the CPU  11  transmits the waveform generation information read from the memory  12  to the signal generator  20  in time series. 
     Operation of Printer  1   
     In the printer  1  having the above configuration, during printing, ink droplets are ejected from the nozzles while a sheet is transported. The ink droplets are landed on the sheet to form dots, and thus an image is formed. 
     Operation of Printer  1  when Ink is Ejected 
     The operation of the printer  1  during ink ejection (hereinafter, also referred to as an ejection operation) will be described. For the ejection operation, the CPU  11  reads out and executes a computer program (firmware) stored in the memory  12 . To this end, firmware has program codes for control related to the ejection operation. 
     First, the CPU  11  analyzes image data to be printed, defines the size (S, M, or L) of a dot to be formed on the sheet, and generates dot forming data in accordance with the defined dot size. In the printer  1 , as shown in  FIG. 5A , dot forming data has two bits per dot. Specifically, when a large dot (L) is formed, the bit values of the dot forming data are set to “11”. When a medium dot (M) is formed, the bit values are set to “10”. When a small dot (S) is formed, the bit values are set to “01”. In addition, when no dot is formed (ink is not ejected), the bit values of dot forming data are set to “00” (described below). Therefore, dot forming data includes information regarding whether or not to form a dot and information for specifying the size of a dot to be formed. The CPU  11  inputs dot forming data to the control signal generating circuit  73 . 
     In the printer  1 , each time the sheet is transported by the amount corresponding to one column (1 dot line) of a unit area arranged in a sheet width direction, the signal level of the latch signal LAT changes. If the signal level changes, the signal generator  20  starts to generate the driving signals (the driving signals COM 1  and COM 2 ). The transport speed of the sheet during printing is uniform. For this reason, the driving signals are repeatedly generated for each repetition cycle T. The generated driving signals COM 1  and COM 2  are input to the head driver  72 . 
     The control signal generating circuit  73  generates the control signal for each piezoelectric element  61  on the basis of dot forming data input from the controller  10 . The generated control signal is output to a pair of switches (the switches  72   a  and  72   b ) corresponding to the piezoelectric element  61 . 
     The control signal assigns a pulse to be applied to each piezoelectric element  61  (hereinafter, also referred to as a pulse to be applied).  FIG. 5B  shows the relationship between dot forming data and the pulse to be applied. Specifically, when the bit values of dot forming data are “11”, the large dot pulse L becomes a pulse to be applied. When the bit values are “10”, the medium dot pulse M becomes a pulse to be applied. When the bit values are “01”, the small dot pulse S becomes a pulse to be applied. 
     A pair of switches operate in accordance with the control signal input from the control signal generating circuit  73 . As a result, the pulse to be applied is applied to the piezoelectric element  61 . The description is provided for the ejection operation, and thus the pulse to be applied is one of the ejection pulses. That is, an ejection pulse is applied to the piezoelectric element  61 . When this happens, an ink droplet in a corresponding amount is ejected from the nozzle, and a dot of a corresponding size (S, M, or L) is formed on the sheet. 
     Such an ejection operation is performed each time the signal level of the latch signal LAT changes. Thus, an image is formed on the sheet. Such control is performed for each pair of switches. 
     Operation of Printer  1  when No Ink is Ejected 
     Next, a case in which an ejection operation is not performed (no dot is formed) will be described. Control when no dot is formed is performed in parallel to the control during the ejection operation. In order to perform parallel control, the bit values of dot forming data can be set to “00” (see  FIG. 5A ). 
     When no dot is formed, the controller  10  generates dot forming data having the bit values “00”, and inputs the generated dot forming data to the control signal generating circuit  73 . When the bit values of dot forming data are “00”, the control signal generating circuit  73  assigns the pulse N for micro vibration as a pulse to be applied (see  FIG. 5B ). Thus, the pulse N for micro vibration is applied to the piezoelectric element  61 . 
     In this case, the piezoelectric element  61  operates in accordance with a potential change pattern of the pulse N for micro vibration. As a result, the pressure of ink in the pressure chamber  63  changes. No ink is ejected from the corresponding nozzle. As will be apparent from  FIG. 4 , this is because the change range of the potential of the pulse N for micro vibration is smaller than the change range of the potential of the ejection pulse, and thus a change in pressure of ink is also small. The change in pressure of ink causes micro vibration of the meniscus. The micro vibration of the meniscus ensures stirring of ink near the nozzle, and thus ink can be prevented from being thickened. 
     Pulse N for Micro Vibration 
     In this embodiment, the pulse N for micro vibration is designed such that the amplitude or duration of micro vibration of the meniscus is optimized. Specifically, the pulse N for micro vibration is designed so as to cause micro vibration of the meniscus at amplitude sufficient to prevent ink droplets from being ejected with irregular timing and prevent ink from being thickened. In addition, the pulse N for micro vibration is designed so as to cause micro vibration of the meniscus with duration sufficient to suppress an adverse effect on the amount of an ink droplet to be ejected from the nozzle and to prevent ink from being noticeably thickened after micro vibration ends. 
     First, the pulse N for micro vibration designed as described above will be described in detail.  FIG. 6  is an enlarged view of the pulse N for micro vibration shown in  FIG. 4  to illustrate a potential change pattern of the pulse N for micro vibration. 
     As shown in  FIG. 6 , the pulse N for micro vibration has a first charging portion N 1 , a first constant potential portion N 2 , a second charging portion N 3 , a second constant potential portion N 4 , and a discharging portion N 5 . The portions N 1  to N 5  are generated during timing t 0  to t 5  (the generation period TN). The portions N 1  and N 2  are connected to each other and are generated during the timing t 0  to t 2  (period Tα). The portions N 2  and N 3  are connected to each other. The portions N 3  and N 4  are connected to each other, and the portions N 4  and N 5  are connected to each other. That is, the portions N 1  to N 5  form a series of potential change pattern. 
     Next, the portions will be described. 
     The first charging portion N 1  corresponds to a line segment AB during a generation period T 1 . The generation period T 1  is a period from the timing t 0  to the timing t 1 . The generation period T 1  is preferably set to be equal to or longer than an intrinsic vibration cycle of the piezoelectric element  61 . In the first charging portion N 1 , a potential V rises from a potential V 1  to a micro vibration medium potential Vm. The potential V is a potential to be input to one terminal of the piezoelectric element  61  (see  FIG. 1 ). In the first charging portion N 1 , the potential of one terminal of the piezoelectric element  61  rises from the potential V 1  to the micro vibration medium potential Vm, thereby charging the piezoelectric element  61 . 
     The potential V 1  is a reference potential preset in the printer  1 , and is an example of a first potential. A potential V 2  is a high potential (a potential difference from the potential V 1 ) set to such an extent that ink is not ejected even if the potential is applied to the piezoelectric element  61 , and is an example of a second potential. The micro vibration medium potential Vm is a predefined potential (described below) between the potential V 1  and the potential V 2  higher than the potential V 1 , and is an example of a medium potential defined between the first potential and the second potential. In this embodiment, the potential V 2  is higher than the potential V 1 , for example, by 5 V, and the micro vibration medium potential Vm is higher than the potential V 1 , for example, by 1 V. Thus, the first charging portion N 1  is an example of a first potential change portion at which a potential changes from the first potential to the medium potential defined between the first potential and the second potential. 
     With respect to the first charging portion N 1 , the absolute value (that is, |Vm−V 1 |) of a difference between the potential V 1  and the micro vibration medium potential Vm is a potential difference ΔVα. That is, the potential difference ΔVα is an example of a difference between the medium potential and the first potential. In this embodiment, a potential change pattern of the first charging portion N 1  is linear, and the slope thereof is constant to be a potential change amount per unit time (ΔVα/T 1 ). 
     The first constant potential portion N 2  corresponds to a line segment BC during the timing t 1  to t 2  (generation period T 2 ). In the first constant potential portion N 2 , the potential V is constant at the micro vibration medium potential Vm. Thus, the first constant potential portion N 2  is an example of a constant potential portion which is generated after the first potential change portion and at which the potential is constant at the medium potential. The first constant potential portion N 2  is used to maintain the piezoelectric element  61  in a predetermined deformation state. 
     The second charging portion N 3  corresponds to a line segment CD during a generation period T 3 . The generation period T 3  is a period from the timing t 2  to the timing t 3 . The generation period T 3  is preferably set to be equal to or longer than the intrinsic vibration cycle of the piezoelectric element  61 . In the second charging portion N 3 , the potential V rises from the micro vibration medium potential Vm to the potential V 2 . Thus, the second charging portion N 3  is an example of a second potential change portion which is generated after the constant potential portion and at which the potential changes from the medium potential to the second potential. In the second charging portion N 3 , the potential V rises from the micro vibration medium potential Vm to the potential V 2 , thereby charging the piezoelectric element  61 . The absolute value (that is, |V 2 −Vm|) of a difference between the micro vibration medium potential Vm and the potential V 2  is a potential difference ΔVβ shown in  FIG. 6 . That is, the potential difference ΔVβ is an example of a difference between the medium potential and the second potential. In this embodiment, a potential change pattern of the second charging portion N 3  is linear, and the slope thereof is constant to be a potential change amount per unit time (ΔVβ/T 3 ). 
     The second constant potential portion N 4  corresponds to a line segment DE during the timing t 3  to t 4  (generation period T 4 ). In the second constant potential portion N 4 , the potential V is constant at the potential V 2 . The second constant potential portion N 4  is used to maintain the piezoelectric element  61  in a predetermined deformation state. 
     The discharging portion N 5  corresponds to a line segment EF during a generation period T 5 . The generation period T 5  is a period from the timing t 4  to the timing t 5 . The generation period T 5  is preferably set to be equal to or longer than the intrinsic vibration cycle of the piezoelectric element  61 . In the discharging portion N 5 , the potential V falls from the potential V 2  to the potential V 1 . In the discharging portion N 5 , the potential V falls from the potential V 2  to the potential V 1 , thereby discharging the piezoelectric element  61 . In this embodiment, a potential change pattern of the discharging portion N 5  is linear, and the slope thereof is constant to be a value defined by a potential change amount per unit time (|V 1 −V 2 |/T 5 ), that is, (V 1 −V 2 )/T 5 . 
     As described above in detail, the pulse N for micro vibration of this embodiment has one constant potential portion (the portion N 2 ) between the generation periods of the two charging portions (the portions N 1  and N 3 ). If the pulse N for micro vibration is applied to the piezoelectric element  61 , micro vibration of the meniscus can be generated. The micro vibration continues with sufficient duration (for example, the generation period TN) in a state where the amplitude is maintained so as to prevent ink from being thickened. This will be described below in detail. 
     State of Ink Before and when Pulse N for Micro Vibration is Applied 
     Next, the state of ink before and when the pulse N for micro vibration is applied to the piezoelectric element  61  will be described with reference to  FIGS. 7A to 7C . 
     First, before the pulse N for micro vibration is applied to the piezoelectric element  61 , the potential V is constant at the potential V 1 . The piezoelectric element  61  is maintained in a deformation state according to the potential V 1 . For this reason, the pressure chamber  63  is kept to a corresponding volume, and no change in pressure occurs in ink filled in the pressure chamber  63 . Therefore, the meniscus is in a stationary state. The state of the meniscus at that time is shown in  FIG. 7A . 
     Next, if the first charging portion N 1  starts to be applied to the piezoelectric element  61 , the piezoelectric element  61  is charged and contracts in an up-down direction (the longitudinal direction of the piezoelectric element  61 ) shown in  FIG. 3 . The contraction causes movement of the vibrating plate  64  in an upper direction in  FIG. 3 , that is, in a direction away from the nozzle. As a result, the volume of the pressure chamber  63  increases. If the volume of the pressure chamber  63  increases, the pressure of ink decreases. For this reason, ink flows into the pressure chamber  63 . In this case, ink flows from the common ink chamber  62   a . If the pressure of ink in the pressure chamber  63  decreases, the meniscus is pulled in toward the pressure chamber  63 , that is, in a direction of an arrow A shown in  FIG. 7B . 
     The pressure chamber  63 , the nozzle, and an ink supply channel (a portion communicating the common ink chamber  62   a  with the pressure chamber  63 ) are formed as a single body and function as an acoustic tube. This is because the pressure chamber  63  corresponds to a flow channel portion having a large sectional area rather than the nozzle or the ink supply channel. Since the pressure chamber  63 , the nozzle, and the ink supply channel are formed as a single body and function as an acoustic tube, when the first charging portion N 1  is applied to the piezoelectric element  61 , pressure vibration of an intrinsic cycle (Helmholtz&#39;s resonance cycle) is applied to ink in the pressure chamber  63 . If pressure vibration is applied to ink in the pressure chamber  63 , the meniscus vibrates in the nozzle. 
     Next, the first constant potential portion N 2  is applied to the piezoelectric element  61 , but the first constant potential portion N 2  causes no change in potential (potential V) on one terminal of the piezoelectric element  61 . For this reason, the piezoelectric element  61  is maintained in a contraction state corresponding to the micro vibration medium potential Vm over the generation period T 2  of the first constant potential portion N 2 . Thus, the volume of the pressure chamber  63  is maintained constant. In this case, the meniscus moves in the nozzle by pressure vibration due to the first charging portion N 1 . 
     Next, the second charging portion N 3  and the second constant potential portion N 4  are sequentially applied to the piezoelectric element  61 . The states of ink at that time are the same as those when the first charging portion N 1  and the first constant potential portion N 2  are applied to the piezoelectric element  61 . Therefore, while the second charging portion N 3  is being applied, the meniscus is pulled in toward the pressure chamber  63 . In addition, while the second constant potential portion N 4  is being applied, the meniscus moves in the nozzle. 
     Finally, the discharging portion N 5  is applied to the piezoelectric element  61 . When this happens, the piezoelectric element  61  is discharged, and the piezoelectric element  61  expands in the longitudinal direction. The expansion causes movement of the vibrating plate  64  toward the pressure chamber  63 . With the movement of the vibrating plate  64 , the meniscus is pushed out in an ejection direction (a direction of an arrow B shown in FIG.  7 C). Thereafter, pressure vibration of ink is attenuated, and the meniscus returns to the state shown in  FIG. 7A . 
     As described above, if the pulse N for micro vibration is applied to the piezoelectric element  61 , the meniscus vibrates in accordance with pressure vibration applied to ink in the pressure chamber  63 . For example, the meniscus repeatedly moves between a state pulled in toward the pressure chamber  63  (a state shown in  FIG. 7B ) and a state pushed out in the ejection direction (a state shown in  FIG. 7C ). 
     Pressure Vibration of Ink 
     Next, pressure vibration to be applied to ink by the pulse N for micro vibration will be described in detail. 
     As described above, if the first charging portion N 1  or the second charging portion N 3  is applied to the piezoelectric element  61 , pressure vibration is applied to ink in the pressure chamber  63 . 
     In this embodiment, a generation start timing of the first charging portion N 1  is different from a generation start timing of the second charging portion N 3 . The generation start timing used herein means a timing at which the portion N 1  or N 3  starts to be applied to the piezoelectric element  61 . In the pulse N for micro vibration of the  FIG. 6 , the timing t 0  or t 2  corresponds to the generation start timing of the portion N 1  or the portion N 3 . In this way, since the generation start timing of the portion N 1  is different from the generation start timing of the portion N 3 , complex pressure vibration occurs in ink of the pressure chamber  63 . 
     For ease of understanding, pressure vibration to be applied to ink in the pressure chamber  63  due to the first charging portion N 1  and pressure vibration to be applied to ink in the pressure chamber  63  due to the second charging portion N 3  are considered separately. 
       FIG. 8A  shows an example of pressure vibration to be applied to ink in the pressure chamber  63  due to the first charging portion N 1 .  FIG. 8A  also shows an example of pressure vibration to be applied to ink in the pressure chamber  63  due to the second charging portion N 3 . In  FIG. 8A , the vertical axis represents an ink pressure in the pressure chamber  61 . The ink pressure is low on an upper side, and is high on a lower side. The horizontal axis represents a time. Therefore, an upward-sloping portion of a line representing pressure vibration indicates the state that the ink pressure decreases as time passes. To the contrary, a downward-sloping portion of the line indicates the state that the ink pressure increases as time passes. 
     A pressure vibration waveform Pα shown in  FIG. 8A  represents pressure vibration applied to ink due to the first charging portion N 1 . The cycle (intrinsic vibration cycle Tc) of the pressure vibration waveform Pα is defined by the structure of the pressure chamber  63 , the material of the vibrating plate  64 , the property of ink, and the like. In this line head  60 , the cycle of the pressure vibration waveform Pα is in a range of approximately 5.5 μs to 6.0 μs. The amplitude of the pressure vibration waveform Pα decreases as time passes. For this reason, amplitude Aα in a first cycle becomes maximum amplitude in the pressure vibration waveform Pα. 
     A pressure vibration waveform Pβ shown in  FIG. 8A  represents pressure vibration applied to ink in the pressure chamber  63  due to the second charging portion N 3 . The cycle of the pressure vibration waveform Pβ is the same as the cycle of the pressure vibration waveform Pα. The amplitude of the pressure vibration waveform Pβ is also attenuated as time passes. For this reason, amplitude Aβ in a first cycle becomes maximum amplitude in the pressure vibration waveform PP. In this embodiment, the maximum amplitude Aβ of the pressure vibration waveform Pβ is set so as to be larger than the maximum amplitude Aα of the pressure vibration waveform Pβ (the details will be described below). 
     Next, a composite waveform of the two pressure vibration waveforms Pβ and Pβ is considered. The composite waveform is shown in  FIG. 8B  as a composite waveform Pm. In  FIG. 8B , the pressure vibration waveforms Pα and Pβ of  FIG. 8A  are indicated by a broken line and a one-dot-chain line, respectively. Like  FIG. 8A , the vertical axis and horizontal axis of the  FIG. 8B  represents an ink pressure and a time, respectively. 
     From a period Tα from timing t 0  to timing t 2 , the second charging portion N 3  is not applied to the piezoelectric element  61 . For this reason, during the period Tα, the composite waveform Pm is identical to the above-described pressure vibration waveform Pα, and the amplitude thereof decreases as time passes. The second charging portion N 3  is applied to the piezoelectric element  61  at the timing t 2 . If the second charging portion N 3  is applied, the pressure vibration waveform Pβ is added to the pressure vibration waveform Pα. For this reason, the composite waveform Pm is different from the pressure vibration waveform Pα after the timing t 2 . 
     That is, the amplitude of the composite waveform Pm increases immediately after the timing t 2 , and then decreases as time passes. The reason why the amplitude of the composite waveform Pm increases immediately after the timing t 2  is that the pressure vibration waveform Pα starting to be attenuated is excited by the pressure vibration waveform Pβ immediately after the timing t 2 . 
     As described above, in this embodiment, the pulse N for micro vibration includes the two charging portions (the portions N 1  and N 3 ) having different generation start timing, thereby exciting pressure vibration starting to be attenuated. This pressure vibration affects on the amplitude of micro vibration of the meniscus. That is, if the pressure vibration is excited, the amplitude of pressure vibration (that is, the amplitude of micro vibration of the meniscus) can be increased. Therefore, even though ink is insufficiently prevented from being thickened, an insufficient effect can be restored such that ink can be sufficiently prevented from being thickened. In addition, if the amplitude of pressure vibration increases, the duration of pressure vibration also increases. 
     In the example shown in  FIG. 8B , excitation by the pressure vibration waveform Pβ starts at the timing t 2 . The timing t 2  is defined in a downward-sloping portion in the pressure vibration waveform Pα. In other words, the timing t 2  is defined during a period in which the ink pressure increases (described below in detail). For this reason, at this timing, the meniscus is pushed out in the ejection direction. Meanwhile, in the pressure vibration waveform Pβ, an upward-sloping portion is present immediately after the timing t 2 . That is, the ink pressure decreases as time passes. In other words, immediately after the timing t 2 , it can be considered that the pressure vibration waveform Pβ is out of phase with the pressure vibration waveform Pα. 
     As described above, since the pressure vibration waveform Pβ is out of phase with the pressure vibration waveform Pα, at the beginning of excitation immediately after the second charging portion N 3  is applied to the piezoelectric element  61 , pressure vibration due to the second charging portion N 3  is slightly weakened by pressure vibration due to the first charging portion N 1 . In other words, the pressure vibration waveform Pβ is out of phase with the pressure vibration waveform Pα such that a change in pressure of ink due to the second charging portion N 3  is weakened by pressure vibration applied to ink due to the first charging portion N 1  when the second charging portion N 3  starts to be applied to the piezoelectric element  61 . Therefore, ink in the pressure chamber  63  can be prevented from being extremely excited. As a result, the amplitude of the composite waveform Pm can be prevented from being unnecessarily increased. 
     The amplitude of the composite waveform Pm indirectly represents the displacement of the meniscus. For this reason, the amplitude of micro vibration of the meniscus can be prevented from being extremely increased. That is, ink can be prevented from being ejected with irregular timing. In this embodiment, the maximum amplitude of the composite waveform Pm is defined so as to be within a range of allowable maximum amplitude Amax. 
     Since the excitation by the pressure vibration waveform Pβ starts at the timing t 2 , the maximum amplitude of the composite waveform Pm may be defined in accordance with the potential difference ΔVβ of the second charging portion N 3 . In setting the potential difference ΔVβ, the maximum amplitude (amplitude Aβ) of the pressure vibration waveform Pβ is preferably set so as to be larger than the maximum amplitude (amplitude Aα) of the pressure vibration waveform Pα. From this viewpoint, as shown in  FIG. 6 , the potential difference ΔVβ is set so as to be larger than the potential difference ΔVα. That is, the micro vibration medium potential Vm (the potential of a point B or C) is set so as to be near the potential V 1 . If the micro vibration medium potential Vm is set in the above-described manner, immediately after the timing t 2 , the amplitude (specifically, maximum amplitude) of the composite waveform Pm can be defined as desired. Thus, the attenuation time of the composite waveform Pm can also be adjusted to a desired length. In order to realize a reliable ink thickening suppression effect by pressure vibration due to the first charging portion N 1 , the potential difference is preferably defined such that the potential difference ΔVα becomes 5% or more of the potential difference (ΔVα+ΔVβ) between the potential V 2  and the potential V 1 , that is, the potential difference ΔVβ is within 95% of the potential difference between the potential V 2  and the potential V 1 . 
     In this embodiment, the generation start timing (timing t 2 ) of the second charging portion N 3  is set starting with the generation start timing (timing t 0 ) of the first charging portion N 1 . That is, the timing t 2  is defined using the intrinsic vibration cycle Tc. This will be described below. 
     The timing t 2  is defined in a section at which the pressure vibration waveform Pα slopes downward, in other words, during a period in which the ink pressure increases. Such a section appears cyclically, and thus a plurality of sections are present. Each section corresponds to a period from one-quarter cycle to three-quarters cycle in each cycle of the intrinsic vibration cycle Tc. For this reason, each section corresponds to a period represented by Expression (3) starting with a start point of a first cycle of the pressure vibration waveform Pα. The start point of the first cycle of the pressure vibration waveform Pα is the timing to.
 
nTc+0.5Tc±0.25Tc  (3)
 
     For Expression (3), n is an integer of “0” or more. 
     Therefore, the timing t 2  is defined within the period represented by Expression (3) starting with the timing t 0 . In the example of  FIG. 8A , the timing t 2  is defined on an assumption that the integer n is “1”. In this case, the timing t 2  can be defined within the t 2  settable period. 
     It is considered that the integer n has an upper limit value. This is because pressure vibration is attenuated as time passes. In this embodiment, the amplitude of the pressure vibration waveform Pα when being attenuated is defined so as not to be smaller than the range Amin shown in  FIG. 8B . The range Amin indicates the boundary of an amplitude range of the pressure vibration in which ink is insufficiently prevented from being thickened. The range Amin is preferably defined on the basis of a degree of attenuation of the pressure vibration waveform Pα. 
     The generation period T 2  in which the first constant potential portion N 2  is generated is defined on the basis of the period Tα. That is, a difference between the period Tα and the generation period T 1  in which the first charging portion N 1  is generated becomes the generation period T 2 . That is, the relationship T 2 =Tα−T 1  is established. The generation period T 2  corresponds to an interval between the first potential change portion and the second potential change portion. 
     Advantages of First Embodiment 
     According to the above-described first embodiment, the pulse N for micro vibration has the first constant potential portion N 2  which is generated between the generation period of the first charging portion N 1  and the generation period of the second charging portion N 3 . For this reason, an interval between the first charging portion N 1  and the second charging portion N 3 , and the micro vibration medium potential Vm can be set. Therefore, the amplitude of the composite waveform Pm or the attenuation time of the composite waveform Pm can be adjusted. The composite waveform Pm causes micro vibration of the meniscus. As a result, the amplitude or duration of micro vibration of the meniscus can be optimized. 
     According to this embodiment, the pressure vibration waveform Pβ, which is a component of the composite waveform Pm, is out of phase with the pressure vibration waveform Pα such that the pressure vibration waveform Pβ is weakened by the pressure vibration waveform Pα, which is another component of the composite waveform Pm, immediately after composition. In other words, the pressure vibration waveform Pβ is out of phase with the pressure vibration waveform Pα such that the change in pressure of ink due to the second charging portion N 3  is weakened by pressure vibration applied to the piezoelectric element  61  due to the first charging portion N 1  when the second charging portion N 3  is applied to the piezoelectric element  61  (at the beginning of excitation). Therefore, pressure vibration applied to ink due to the first charging portion N 1  can be prevented from being extremely excited immediately after the timing t 2 . The timing t 2  is defined within the t 2  settable period on the basis of the intrinsic vibration cycle Tc by Expression (3). 
     According to this embodiment, the potential difference ΔVβ is larger than the potential difference ΔVα. Therefore, the attenuation time of the composite waveform Pm, that is, the duration of micro vibration of the meniscus can be appropriately adjusted. 
     Timing t 4   
     In this embodiment, like the timing t 2 , the timing t 4  (the total time of the generation period T 3  and the generation period T 4 ) is defined within the range represented by Expression (3). However, the timing t 4  is defined starting with the timing t 2 , not the timing t 0 . If the timing t 4  is defined in the above-described manner, the pressure vibration waveform Pβ is easily in phase with the pressure vibration waveform Pα, and thus vibration of the meniscus can be efficiently suppressed. As a result, when a next ejection pulse (large dot pulse L) is applied to the piezoelectric element  61 , there is no case in which vibration of the meniscus caused by application of the pulse N for micro vibration to the piezoelectric element  61  remains (residual vibration). Therefore, there is no influence on the amount of an ink droplet to be ejected from the nozzle. As a result, a variation in the amount of an ink droplet to be ejected can be eliminated, and thus an ink droplet can be stably ejected. 
     Adjacent Crosstalk 
     In this embodiment, as the driving signal for driving the piezoelectric element  61 , the two driving signals COM 1  and COM 2  are used. Therefore, a plurality of pulses can be generated at the repetition cycle T. In this way, dot formation for one dot line can be speeded up. 
     However, when the generation periods of a plurality of pulses at the repetition cycle T overlap each other, a variation in the amount of an ink droplet to be ejected from the nozzle may occur due to the adjacent crosstalk phenomenon. The adjacent crosstalk phenomenon occurs between adjacent pressure chambers  63  and  63 , and means the phenomenon that a change in pressure of one pressure chamber  63  propagates through the partition wall and has an affect on the ink pressure of the other pressure chamber  63 . 
     It is assumed that the pulse for micro vibration is generated at the first half of the repetition cycle T, and the small dot pulse is generated at the second half of the repetition cycle T. In this case, if pressure vibration applied to ink by the pulse for micro vibration is extremely large, a change in pressure propagates an adjacent pressure chamber  63 , and a variation in the amount of an ink droplet for a small dot to be ejected may occur. From this viewpoint, if the pulse N for micro vibration of this embodiment is used, the amplitude of pressure vibration can be suppressed, and the pressure vibration can be maintained for a long time. Therefore, an influence of adjacent crosstalk on the adjacent pressure chamber  63  can be suppressed. For example, a variation in the amount of an ink droplet for a small dot to be ejected can be suppressed. 
     Change Signal CH 
     The CPU  11  generates the change signal CH 2  within the generation period (a period Tflat of  FIG. 4 ) of the constant potential portion (specifically, the first constant potential portion N 2 ) of the pulse N for micro vibration in the generation period of the pulse N for micro vibration. In this way, if the change signal CH 2  is generated within the generation period of the constant potential portion of the pulse N for micro vibration, while the pulse N for micro vibration is being applied to the piezoelectric element  61 , an influence (noise) of a pulse due to switching of the change signal CH 2  can be substantially eliminated. The change signal is used when the control signal generating circuit  72  switches the switches  72   a  and  72   b . During the switching operation of the switches, noise easily occurs in the driving signal. 
     Similarly, in order to substantially eliminate an influence (noise) of a pulse due to switching of the change signal CH 1  on the medium dot pulse M while the medium dot pulse M is being applied to the piezoelectric element  61 , the CPU  11  generates the change signal CH 1  within the generation period of the constant potential portion of the medium dot pulse M. 
     Method of Setting Pulse N for Micro Vibration 
     The potential change pattern of the pulse N for micro vibration is defined in accordance with the waveform generation information stored in the memory  12  in advance. In other words, the potential information (digital data) of the pulse N for micro vibration is set when the waveform generation information corresponding to the driving signals COM 1  and COM 2  is stored in the memory  12  of the printer  1  or when the waveform generation information written in the memory  12  in advance is overwritten. With respect to settings, the potential information of the potential points A, B, C, D, E, and F shown in  FIG. 6  may be recorded in the memory  12 , together with information regarding time series. Therefore, at least the following potential information is set: potential information required for changing the potential V from the potential V 1  to the micro vibration medium potential Vm defined between the potential V 1  and the potential V 2 ; potential information required for maintaining the potential V constant at the micro vibration medium potential Vm after the potential V has changed from the potential V 1  to the micro vibration medium potential Vm; and potential information required for changing the potential V from the micro vibration medium potential Vm to the potential V 2  after the potential V has been maintained at the micro vibration medium potential Vm are set. In this way, the generation period of the first constant potential portion N 2  (the interval between the first charging portion N 1  and the second charging portion N 3 ), and the micro vibration medium potential Vm are appropriately set. Therefore, when the printer  1  is used, the amplitude or attenuation time of pressure vibration of ink in the pressure chamber  63  can be adjusted, thereby optimizing the pulse N for micro vibration. 
     Second Embodiment 
     Next, a second embodiment of the invention will be described. 
     In this embodiment, the same printer  1  as that in the first embodiment is used. However, in this embodiment, instead of the pulse N for micro vibration in the driving signal COM 1  of the first embodiment, a pulse N′ for micro vibration is generated (set). For this reason, while the configuration of the printer and the constituent elements of the printer will be omitted, the pulse N′ for micro vibration will be described in detail. 
       FIG. 9  is a diagram illustrating a potential change pattern of the pulse N′ for micro vibration according to this embodiment.  FIG. 10  is a diagram illustrating pressure vibration to be applied ink in the pressure chamber  63  when the pulse N′ for micro vibration shown in  FIG. 9  is applied to the piezoelectric element  61 . 
     The pulse N′ for micro vibration shown in  FIG. 9  is substantially the same as the pulse N for micro vibration shown in  FIG. 6 , and has the same portions as those of the pulse N for micro vibration in  FIG. 6 . Therefore, in this embodiment, pressure vibration applied to ink due to the first charging portion N 1 ′ is excited by pressure vibration applied to ink due to the second charging portion N 3 ′. However, the pulse N′ for micro vibration has the generation start timing (excitation timing) of the second charging portion and the value (that is, the potential difference) of the micro vibration medium potential defined between the potential V 1  and the potential V 2  different from the pulse N for micro vibration. For this reason, excitation of a waveform starting to be attenuated is different from that described in the first embodiment. 
     First, the excitation timing will be described. 
     As shown in  FIG. 10 , in this embodiment, the excitation timing is timing t 2 ′. The timing t 2 ′ is defined in a portion at which the pressure vibration waveform Pα′ due to the first charging portion N 1 ′ slopes upward (during a period in which the ink pressure decreases). Therefore, the pressure vibration waveform Pβ′ due to the second charging portion N 3 ′ is in phase with the pressure vibration waveform Pα′ (see arrows A′ and B′). 
     As described above, since the pressure vibration waveform Pβ′ is in phase with the pressure vibration waveform Pα′, at the beginning of excitation immediately after the second charging portion N 3 ′ is applied to the piezoelectric element  61 , pressure vibration due to the second charging portion N 3 ′ is slightly strengthened by pressure vibration due to the first charging portion N 1 ′. In other words, the phase of the change in pressure of ink due to the second charging portion N 3 ′ is set such that the change in pressure is strengthened by pressure vibration applied to ink due to the first charging portion N 1 ′ when the second charging portion N 3 ′ starts to be applied to the piezoelectric element  61 . Therefore, at the beginning of application, the pressure vibration waveform Pα′ can be efficiently excited by the pressure vibration waveform Pβ′. As a result, the amplitude of the composite waveform Pm can be easily increased. 
     Next, the potential difference will be described. 
     In this embodiment, excitation by the pressure vibration waveform Pβ′ starts with the timing t 2 ′. For this reason, the maximum amplitude of the composite waveform Pm′ can be defined in accordance of the potential difference ΔVβ′ of the second charging portion N 3 ′. In this embodiment, excitation is efficiently performed, and thus it is not necessary to set the potential difference ΔVβ′ such that the maximum amplitude (amplitude Aβ′) of the pressure vibration waveform Pβ′ is larger than the maximum amplitude (amplitude Aα′) of the pressure vibration waveform Pα′. 
     In this embodiment, the micro vibration medium potential Vm′ is defined near the potential V 2  such that the potential difference ΔVβ′ between the micro vibration medium potential Vm′ and the potential V 2  is smaller than the potential difference ΔVα′ between the potential V 1  and the micro vibration medium potential Vm′ (see  FIG. 9 ). In this case, in order to realize a reliable ink thickening suppression effect due to the second charging portion N 3 ′, the potential difference is preferably defined such that the potential difference ΔVβ′ becomes 5% or more of the potential difference (ΔVα′+ΔVβ′) between the potential V 2  and the potential V 1 , that is, the potential difference ΔVα′ is within 95% of the potential difference between the potential V 2  and the potential V 1 . 
     If the potential difference is set in such a manner, the amplitude of pressure vibration to be applied to ink due to each of the first charging portion N 1 ′ and the second charging portion N 3 ′ can be optimized. 
     Next, a way to define the timing t 2 ′ will be described. 
     In this embodiment, as described above, the timing t 2 ′ is defined in an upward-sloping portion in the pressure vibration waveform Pα′. The upward-sloping portion in the pressure vibration waveform Pα′ is represented by Expression (4) starting with the first cycle (timing t 0 ) of the intrinsic vibration cycle Tc.
 
mTc±0.25Tc  (4)
 
     For Expression (4), m is an integer of “0” or more. Like the above-described integer n, the range of a usable value is defined in accordance with the amplitude Amax or the range Amin. 
     Therefore, the timing t 2 ′ (that is, the period Tα′) is defined within the period represented by Expression (4) starting with the timing t 0 . In the example of  FIG. 10 , the timing t 2 ′ is defined on an assumption that the integer m is “2”. In this case, the timing t 2 ′ can be defined within the t 2 ′ settable period. 
     The generation start timing (timing t 4 ′) of the discharging portion N 5 ′ is defined using Expression (3), like the first embodiment. 
     As described above in detail, according to the second embodiment, the pulse N′ for micro vibration has the first constant potential portion N 2 ′ which is generated between the generation period of the first charging portion N 1 ′ and the generation period of the second charging portion N 3 ′. For this reason, like the first embodiment, the amplitude of the composite waveform Pm′ or the attenuation time of the composite waveform Pm′ can be adjusted. 
     According to this embodiment, the phase of the pressure vibration waveform Pβ′, which is one component of the composite waveform Pm′, is set such that the pressure vibration waveform Pβ′ is strengthened by the pressure vibration waveform Pα′, which is another component of the composite waveform Pm′, immediately after composition. In other words, the phase of the change in pressure of ink due to the second charging portion N 3 ′ is set such that the change in pressure is strengthened by pressure vibration applied to the piezoelectric element  61  due to the first charging portion N 1 ′ when the second charging portion N 3 ′ starts to be applied to the piezoelectric element  61  (at the beginning of excitation). Therefore, even if pressure vibration applied to ink due to the first charging portion N 1 ′ starts to be attenuated, the pressure vibration can be efficiently excited. The timing t 2 ′ is defined within the t 2 ′ settable period, which is defined on the basis of the intrinsic vibration cycle Tc by Expression (4). 
     According to this embodiment, the potential difference ΔVβ′ is smaller than the potential difference ΔVα′. Therefore, the amplitude of pressure vibration applied to ink due to the first charging portion N 1 ′ and the second charging portion N 3 ′ can be optimized. 
     Other Embodiments 
     Pulse for Micro Vibration 
     In the foregoing first and second embodiments, the pulses N and N′ for micro vibration each include the two charging portions. Alternatively, the pulse for micro vibration may include three or more charging portions.  FIG. 11  shows a pulse for micro vibration having three charging portions. In this case, as shown in  FIG. 11 , the pulse for micro vibration preferably has a constant potential portion between two charging portions. 
     When the pulse for micro vibration include three or more charging portions, the excitation timing by another charging portion which is generated after one charging portion is set in the same manner as described in the foregoing first or second embodiment. For example, the excitation timing by the second charging portion is set in the same manner as described in the first embodiment (or the second embodiment), and the excitation timing by the third charging portion is set in the same manner as described in the second embodiment (or the first embodiment). The method of setting the excitation timing is not limited thereto. For example, the excitation timing by the second and third charging portions may be set in the same manner as described in the first embodiment (or the second embodiment). 
     In the foregoing first and second embodiments, the pulses N and N′ for micro vibration each include one constant potential portion between two charging portions. Alternatively, the pulse for micro vibration may include no constant potential portion.  FIG. 12A  shows a pulse for micro vibration in which the constant potential portion (the first constant potential portion N 2 ) in the pulse N for micro vibration of  FIG. 6  is not provided between the two charging portions. In this way, if the pulse for micro vibration has two charging portions, the vibration of the meniscus starting to be attenuated can be excited. However, the two charging portions of the pulse for micro vibration are different in the potential change amount per unit time. 
     Although in the foregoing first and second embodiments, a case in which the potential change pattern of the charging portion is linear (line segment) has been described, the potential change pattern of the charging portion may be curved.  FIG. 12B  shows a case in which the potential change pattern of each charging portion of the pulse for micro vibration shown in  FIG. 12A  is curved. 
     Although in the foregoing first and second embodiments, the two driving signals COM 1  and COM 2  are generated as the driving signal, a single driving signal may be used.  FIG. 13  shows a driving signal COM having the pulse N for micro vibration of the driving signal COM 1  and the small dot pulse S of the driving signal COM 2  shown in  FIG. 4 . 
     Although the potential V 2  is higher than the potential V 1 , for example, by 5 V, the potential difference is not limited to 5 V. When ink is aqueous ink, the potential difference may be set to 5 V, and when ink is pigment ink or dye ink, the potential difference may be set to be in the range of 5 to 8 V. In this way, the potential difference may be appropriately changed. 
     In the foregoing first embodiment, the potential difference of the second charging portion is larger than the potential difference of the first charging portion. In addition, in the foregoing second embodiment, the potential difference of the second charging portion is smaller than the potential difference of the first charging portion. Alternatively, the potential difference of the second charging portion may be equal to the potential difference of the first charging portion. In this case, the pressure vibration applied to ink due to the first charging portion and the pressure vibration applied to ink due to the second charging portion have appropriate amplitude. For this reason, the excitation timing by the second charging portion may be defined within the range represented by Expression (3) or may be defined within the range represented by Expression (4). 
     Generation Start Timing of Discharging Portion 
     In the first and second embodiments, the generation start timing (timing t 4  or t 4 ′) of the discharging portion is defined using the intrinsic vibration cycle Tc. Alternatively, the cycle of the pressure vibration waveform (composite waveform) defined by the generation start timing of the second charging portion may be predicted (simulation), and the generation start timing of the discharging portion may be defined using the predicted cycle (or the phase). This is because at the beginning of composition, the cycle of the composite waveform is not constant and out of the intrinsic vibration cycle Tc in accordance with the ratio of the pressure vibration waveform as a component. For example, in the case of the composite waveform Pm shown in  FIG. 8B , the cycle is slightly longer than the intrinsic vibration cycle Tc (if the ratio of the pressure vibration waveform Pβ as a component increases, the cycle of the composite waveform Pm becomes identical to the intrinsic vibration cycle Tc). 
     Piezoelectric Element  61   
     The intrinsic vibration cycle of the piezoelectric element  61  is preferably shorter than the intrinsic vibration cycle Tc of pressure vibration applied to ink due to the charging portion or discharging portion of the pulse for micro vibration. 
     Although in the foregoing embodiments, a case in which the piezoelectric element  61  is charged and the volume of the pressure chamber  63  increases has been described, the same description is applied to a case in which the piezoelectric element  61  is discharged and the volume of the pressure chamber  63  increases. 
     In the foregoing embodiments, instead of the piezoelectric elements  61 , for example, magnetostrictive elements may be used. 
     Printer  1   
     In the foregoing embodiments, the printer  1  ejects ink droplets from the line head  60  while transporting the sheet. However, the invention may be applied to a serial printer that performs printing while moving a head ejecting ink droplets. 
     Liquid Ejecting Apparatus 
     In the foregoing embodiments, the printer  1  in which ink is used as the liquid filled in the ink flow channel  62  including the pressure chambers  63  has been described. However, the liquid filled in the ink flow channel  62  is not limited to ink. Specific examples of the liquid ejecting apparatus include a liquid ejecting apparatus that ejects a liquid, in which a material, such as an electrode material or a color material, is dispersed or dissolved, and is used in manufacturing a liquid crystal display, an EL (Electro Luminescence) display, and a field emission display, a liquid ejecting apparatus that ejects a bioorganic material to be used in manufacturing a bio-chip, a liquid ejecting apparatus that ejects a liquid (sample) as a precision pipette. In addition, a liquid ejecting apparatus that pinpoint ejects lubricant to a precision instrument, such as a watch or a camera, a liquid ejecting apparatus that ejects on a substrate a transparent resin liquid, such as ultraviolet cure resin, to form a fine hemispheric lens (optical lens) for an optical communication element, a liquid ejecting apparatus that ejects an etchant, such as acid or alkali, to etch a substrate or the like, and a liquid ejecting apparatus that ejects gel may be used. The invention may be applied to one of the liquid ejecting apparatuses.