Patent Publication Number: US-2023142144-A1

Title: Liquid droplet discharge apparatus

Description:
REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from Japanese Patent Application No. 2021-180831 filed on Nov. 5, 2021. The entire content of the priority application is incorporated herein by reference. 
     BACKGROUND ART 
     Conventionally, there is a technique for detecting whether ink is discharged or not from a nozzle provided on a discharge head. For example, an apparatus which determines whether ink is discharged or not based on a current value detected at an electrode arranged facing the discharge head is known. According to this configuration, it is possible to determine, at the same time, whether ink droplets are discharged or not from a plurality of nozzles. 
     DESCRIPTION 
     However, there is such a case that a piezoelectric element deteriorates as the drive time thereof passes, in a preceding stage before the nozzle reaches a non-discharge or discharge failure, due to which the volume of ink droplet changes. Therefore, the discharge accuracy of each of the nozzles is lowered, which in turn leads to such a fear that any desired image quality cannot be obtained. 
     In view of the above-described situation, an object of the present disclosure is to provide a liquid droplet discharge apparatus capable of detecting a discharge malfunction before the nozzles reach the non-discharge. 
     According to an aspect of the present disclosure, there is provided a liquid droplet discharge apparatus including: 
     a metallic discharge head configured to discharge a liquid droplet onto a print medium; 
     an electrode arranged facing the discharge head; 
     a voltage source configured to generate a potential difference between the discharge head and the electrode; 
     a current detection part configured to detect a current flowing between the discharge head and the electrode; and 
     a controller, 
     wherein the controller is configured to calculate a volume of the liquid droplet based on a value of the current, which is detected by the current detection part under a condition that the liquid droplet is discharged from the discharge head in a state of the potential difference being generated between the discharge head and the electrode by the voltage source. 
     According to the present disclosure, by calculating the volume of the liquid droplet by the controller, it is possible to determine or discriminate, based on the volume of the liquid droplet, whether or not the piezoelectric element has deteriorated in the preceding stage before the nozzle(s) reaches the non-discharge. With this, it is possible to perform a correction for adjusting the volume of the liquid droplet based on the result of determination. Accordingly, it is possible to suppress any decrease in the discharge accuracy of each of the plurality of nozzles and to obtain a desired image quality. 
     According to the present disclosure, it is possible to provide a liquid droplet discharge apparatus capable of detecting the discharge malfunction before the nozzles reach the non-discharge. 
    
    
     
         FIG.  1    is a plan view schematically depicting a configuration of a liquid droplet discharge apparatus. 
         FIG.  2    is a cross-sectional view depicting a configuration of a discharge head in the liquid droplet discharge apparatus of  FIG.  1   . 
         FIG.  3    is a plan view depicting a plurality of nozzles arranged side by side in an alignment direction. 
         FIG.  4    is a block diagram depicting the configuration of the liquid droplet discharge apparatus of  FIG.  1   . 
         FIG.  5    is a plan view depicting a shape of an electrode in a purge unit. 
         FIG.  6    is a view for explaining a method of calculating a volume of an ink droplet in a case that the ink droplet is separated from a nozzle surface. 
         FIG.  7    is a view of discharge waveforms before and after a correction. 
         FIG.  8    is a view for explaining a method for calculating a volume of an ink droplet extending in a discharge direction under a condition that the ink droplet is separated from the nozzle surface. 
         FIG.  9    is a view for explaining a method for calculating an electric field strength of a needle electrode. 
     
    
    
     In the following, a liquid droplet discharge apparatus according to an embodiment of the present disclosure will be explained with reference to the drawings. The liquid droplet discharge apparatus described below is merely an embodiment of the present disclosure. Therefore, the present disclosure is not limited to or restricted by the following embodiment, and any addition, deletion and/or modification can be made with respect to the present disclosure, without departing from the spirit of the present disclosure. 
     First Embodiment 
     As depicted in  FIG.  1   , a liquid droplet discharge apparatus  10  of the present embodiment is configured to discharge an ink droplet as an example of a liquid droplet, and is provided with: a storage tank  12 , a carriage  16 , a metallic discharge head  20 , a pair of conveying rollers  15 , a pair of guide rails  17 , a sub tank  18 , and a purge unit  83  corresponding to a maintenance unit. Further, in the liquid droplet discharge apparatus  10 , a print medium W is placed on a platen which is not depicted. 
     The discharge head  20  is mounted on the carriage  16 . The pair of guide rails  17  extends in a main scanning direction which is orthogonal to a conveying direction of the print medium W. The carriage  16  is supported by the pair of guide rails  17  and reciprocates in the main scanning direction along the guide rails  17 . As a result, the discharge head  20  reciprocates in the main scanning direction. Further, for example, four pieces of the sub tank  18  are mounted on the carriage  16 . Each of the sub-tanks  18  is connected to the storage tank  12  corresponding thereto, via a tube. 
     The pair of conveying rollers  15  are arranged parallel to each other along the main scanning direction. The conveying rollers  15  rotate in a case that a conveying motor which is not depicted is driven, whereby the print medium W placed on the platen is conveyed in the conveying direction. 
     An ink is stored in the storage tank  12 . The storage tank  12  is connected to the discharge head  20  via an ink channel in order to supply ink to the discharge head  20 . Further, the storage tank  12  is provided for each type of the ink. For example, the storage tank  12  is provided as four storage tanks  12 , and black, yellow, cyan, and magenta inks are stored in the four storage tanks  12 , respectively. 
     In the purge unit  83 , a purge processing for forcibly discharging ink from the nozzles  21 , which will be described later on, is performed. Further, an electrode  84  ( FIG.  5   , etc.) is provided on the purge unit  83 . Note that the details of the electrode  84  will be described later. 
     The discharge head  20  moves in the main scanning direction orthogonal to the conveying direction. As depicted in  FIG.  2   , the discharge head  20  has a plurality of nozzles  21  configured to discharge or eject the inks. The discharge head  20  has a stacked body including a liquid channel forming body and a volume changing part. Inside the liquid channel forming body, a liquid channel is formed. A plurality of nozzle holes  21   a  are opened in a nozzle surface  40   a  which is a lower surface of the liquid channel forming body. The volume of the liquid channel is changed by driving the volume changing part. At this time, a meniscus vibrates in the nozzle hole  21   a  and the ink droplet is discharged. 
     As depicted in  FIG.  2   , the above-mentioned channel forming body of the discharge head  20  is a stacked body or laminated body of a plurality of plates; and the volume changing part includes a vibration plate  55  and an actuator (piezoelectric element)  60 . An insulating film  56  is connected on the vibration plate  55 , and a common electrode  61 , which will be described later, is connected on the insulating film  56 . 
     The plurality of plates includes, in order from the lower part, a nozzle plate  46 , a spacer plate  47 , a first channel plate  48 , a second channel plate  49 , a third channel plate  50 , a fourth channel plate  51 , a fifth channel plate  52 , a sixth channel plate  53 , and a seventh channel plate  54 , and these plates are stacked on one another. The first channel plate  48 , the second channel plate  49 , the third channel plate  50 , the fourth channel plate  51 , and the fifth channel plate  52  construct a plate  44  for manifold. 
     Holes and grooves of various sizes are formed in each of the plates. Inside the liquid channel forming body in which the respective plates are stacked, the holes and the grooves are combined to thereby form the plurality of nozzles  21 , a plurality of individual channels  64  and a manifold  22 , as the liquid channel. 
     Each of the plurality of nozzles  21  is formed so as to penetrate the nozzle plate  46  in a stacking direction. By arranging or aligning the plurality of nozzle holes  21   a  in an arrangement direction in the nozzle surface  40   a  of the nozzle plate  46 , a nozzle array is formed in the nozzle surface  40   a , as depicted in  FIG.  3   . The arrangement direction is a direction orthogonal to the stacking direction. 
     The manifold  22  supplies the ink to a pressure chamber  28 , which will be described later, to which an ink discharge pressure is applied. The manifold  22  extends in the arrangement direction and is connected to an end of each of the plurality of individual channels  64 . That is, the manifold  22  functions as a common channel for the ink. The manifold  22  is formed by overlapping, in the stacking direction, a through hole penetrating from the first channel plate  48  to the fourth channel plate  51  in the stacking direction and a recess recessed from the lower surface of the fifth channel plate  52 . 
     The nozzle plate  46  is arranged at a location below the spacer plate  47 . The spacer plate  47  is made of, for example, stainless steel. The spacer plate  47  is formed with a recess part  45  recessed, for example, by half etching, in a thickness direction of the spacer plate  47  from a surface, of the space plate  47 , which is on the side of the nozzle plate  46 . The recess part  45  has a thin part forming a damper part  47   a  and a damper space  47   b . With such a configuration, the damper space  47   b  as a buffer space is defined between the manifold  22  and the nozzle plate  46 . 
     A supply port  22   a  communicates with the manifold  22 . The supply port  22   a  is formed, for example, to have a cylindrical shape, and is provided at one end in the arrangement direction of the manifold  22  (the longitudinal direction of the manifold  22 ). Note that the manifold  22  and the supply port  22   a  are connected by a channel which is not depicted and which is provided by penetrating each of an upper part of the fifth channel plate  52 , the sixth channel plate  53 , and the seventh channel plate  54 . 
     Each of the plurality of individual channels  64  is connected to the manifold  22 . In each of the plurality of individual channels  64 , an upstream end thereof is connected to the manifold  22  and a downstream end thereof is connected to a base end of the nozzle  21 . Each of the plurality of individual channels  64  is constructed by a first communication hole  25 , a supply throttle channel  26  which is an individual throttle channel, a second communication hole  27 , a pressure chamber  28 , and a descender  29 , and these constituent components are connected in this order. 
     A lower end of the first communication hole  25  is connected to an upper end of the manifold  22 . The first communication hole  25  extends from the manifold  22  in an upward orientation of the stacking direction, and penetrates an upper part of the fifth channel plate  52  in the stacking direction. 
     An upstream end of the supply throttle channel  26  is connected to an upper end of the first communication hole  25 . The supply throttle channel  26  is formed by, for example, half-etching, and is constructed of a groove recessed from a lower surface of the sixth channel plate  53 . Further, an upstream end of the second communication hole  27  is connected to a downstream end of the supply throttle channel  26 . The second communication hole  27  is formed so as to extend from the supply throttle channel  26  in the upward orientation of the stacking direction, and to penetrate the sixth channel plate  53  in the stacking direction. 
     An upstream end of the pressure chamber  28  is connected to a downstream end of the second communication hole  27 . The pressure chamber  28  is formed by penetrating the seventh channel plate  54  in the stacking direction. 
     The descender  29  is formed by penetrating the spacer plate  47 , the first channel plate  48 , the second channel plate  49 , the third channel plate  50 , the fourth channel plate  51 , the fifth channel plate  52 , and the sixth channel plate  53  in the stacking direction. The descender  29  is arranged, with respect to the manifold  22 , on one side in a width direction, which is orthogonal to the arrangement direction (the left side in  FIG.  2   ). An upstream end of the descender  29  is connected to a downstream end of the pressure chamber  28 , and a downstream end of the descender  29  is connected to the base end of the nozzle  21 . The nozzle  21  overlaps, for example, the descender  29  in the stacking direction, and is arranged in the center, of the descender  29 , in the direction orthogonal to the stacking direction (the width direction). 
     The vibration plate  55  is stacked on the seventh channel plate  54  and covers an upper end opening of the pressure chamber  28 . 
     The actuator  60  includes the common electrode  61 , a piezoelectric layer  62  and individual electrodes  63 . The common electrode  61 , the piezoelectric layer  62  and the individual electrodes  63  are arranged in this order from the lower side. The common electrode  61  covers the entire surface of the vibration plate  55  via the insulating film  56 . The piezoelectric layer  62  covers the entire surface of the common electrode  61 . Each of the individual electrodes  63  is to correspond to one piece of the pressure chamber  28 , and the individual electrodes  63  are arranged on the piezoelectric layer  62 . One piece of an actuator  60  is constructed by one piece of the individual electrode  63 , the common electrode  61 , and a part, of the piezoelectric layer  62 , which is sandwiched between both of the individual electrode  63  and the common electrode  61  (an active part of the piezoelectric layer  62 , to be described later on). 
     Each of the individual electrodes  63  is electrically connected to a driver IC. The driver IC receives a control signal from a controller  71  which will be described later; and the driver IC generates a drive signal, and applies the drive signal to each of the individual electrodes  63 . On the other hand, the common electrode  61  is always held at the ground potential. In such a configuration, an active part of the piezoelectric layer  62  expands and contracts in a plane direction together with the two electrodes  61  and  63  in accordance with the drive signal. In response to this, the vibration plate  55  deforms in a direction in which the vibration plate  55  increase or decrease the volume of a certain pressure chamber  28  which corresponds to the certain individual electrode  63 . As a result, a discharge pressure for discharging the ink droplet from a certain nozzle  21  which corresponds to the certain pressure chamber  28  is applied to the ink in the certain pressure chamber  28 . 
     In the discharge head  20  as described above, the supply port  22   a  is connected to the sub tank  18  via a pipe. In a case that a pressurizing pump provided on the pipe is driven, the ink flows from the sub tank  18  through the pipe and flows into the manifold  22  via the supply port  22   a . Then, the ink flows from the manifold  22  into the supply throttle channel  26  via the first communication hole  25 , and flows from the supply throttle channel  26  into the pressure chamber  28  via the second communication hole  27 . Then, the ink flows through the descender  29  and flows into the nozzle  21 . Here, in a case that the discharge pressure is applied to the ink in the pressure chamber  28  by the actuator  60 , the ink droplets are discharged from the nozzle hole  21   a.    
     As depicted in  FIG.  4   , in addition to the above constituent components, the liquid droplet discharge apparatus  10  includes the controller  71  constructed of a CPU, etc., a RAM  72  and a ROM  73  each corresponding to a storage part, a head driver IC  74 , a waveform generation circuit  76 , a voltage source  80 , a current detection part  81 , motor driver ICs  30  and  32 , a conveying motor  31 , and a carriage motor  33 . 
     The voltage source  80  generates a potential difference between the discharge head  20  and the electrode  84  in accordance with an instruction of the controller  71 . The current detection part  81  detects a current flowing between the discharge head  20  and the electrode  84  in a case that the potential difference is generated between the discharge head  20  and the electrode  84  by the voltage source  80 . 
     The controller  71  receives a result of the detection by the current detection part  81 . The controller  71  causes the discharge head  20  to discharge an ink droplet Id ( FIG.  6   ) in a state that the voltage source  80  generates the potential difference between the discharge head  20  and the electrode  84 . Then, the controller  71  calculates the volume of the ink droplet Id based on a current value (a value of the current), which is detected by the current detection part  81  under a condition that the ink droplet Id is discharged. The details of a method of calculating the volume of the ink droplet Id will be described later. 
     The RAM  72  stores discharge data, etc. Further, the RAM  72  previously stores, as a reference value, the product of a value of the current to be detected by the current detection part  81  in a case that the volume of the ink droplet Id is normal and a time during which the current flows. The ROM  73  stores a liquid droplet discharge program, a control program for performing various data processing, etc. 
     The head driver IC  74  receives the instruction from the controller  71  and causes the discharge head  20  to discharge the ink droplet Id. The motor driver IC  30  receives an instruction from the controller  71  and performs drive control of the conveying motor  31 . The conveying motor  31  conveys the print medium W in the conveying direction by operating the conveying roller  15 . Further, the motor driver IC  32  receives an instruction from the controller  71  and performs drive control of the carriage motor  33 . The carriage motor  33  moves the discharge head  20  in the main scanning direction by operating the carriage  16 . 
       FIG.  5    is a plan view depicting a shape of the electrode  84  in the purge unit  83 . As depicted in  FIG.  5   , the electrode  84  is provided, for example, as electrodes  84  arranged at the four corners, respectively, in the purge unit  83 . The electrodes  84  are arranged so as to face the discharge head  20  in a case that the discharge head  20  is moved into the purge unit  83 . The shape of the electrode  84  is not limited to the above example, provided that a potential difference can be generated between the discharge head  20  and the electrode  84  by the voltage source  80 . As another example of the electrode  84 , for example, the electrode  84  may be formed in a cross shape in a plan view. 
     Next, the method of calculating the volume of the ink droplet Id by the controller  71  will be explained with reference to the drawings. 
     In a case that the ink droplet Id is to be discharged by the discharge head  20  in the state that the potential difference is generated between the discharge head  20  and the electrode  84  by the voltage source  80 , electric charges corresponding to the charge amount of the ink droplet Id are induced on the discharge head  20  and the electrode  84 , respectively. Therefore, a current corresponding to a difference between the charge amount induced on the discharge head  20  and the charge amount induced on the electrode  84  flows between the discharge head  20  and the electrode  84 . At this time, the current detection part  81  detects the current flowing between the discharge head  20  and the electrode  84 . Here, the charge amount of the ink droplet Id is a charge amount corresponding to the volume of the ink droplet Id. Further, the current value detected by the current detection part  81  is proportional to the charge amount of the ink droplet Id. Therefore, as the volume of the ink droplet Id increases, the current value increases. The volume of the ink droplet Id can be obtained by detecting the current value by the current detecting part  81 . 
     As depicted in  FIG.  6   , in a case that the ink droplet Id from the discharge head  20  is discharged from the nozzle surface  40   a  in the state that the potential difference is generated between the discharge head  20  and the electrode  84  by the voltage source  80 , a distance d1 from the nozzle surface  40   a  to a tip position Ps of the ink droplet Id in a discharge direction Dt is less than a threshold value. This threshold is determined previously in accordance with the type (kind) of the ink. At this time, the charge amount Q induced on the nozzle surface  40   a  by the ink droplet Id is represented by a calculation formula: Q=k′×ε×(S/d)×V1. Therefore, the controller  71  calculates the charge amount Q by the calculation formula: Q=k′×F×(S/d)×V1. In the above-described calculation formula, k′ is a predetermined coefficient, P is the permittivity of air, S is a cross-sectional area of the ink droplet Id, d is the distance between the nozzle surface  40   a  and the electrode  84 , and V1 is a voltage applied by the voltage source  80 . 
     Here, in a case that k is a predetermined coefficient, it is possible to derive a calculation formula: S=k×Q, in accordance with the above-described calculation formula. Since the cross-sectional area of the ink droplet Id is expressed by S=π×r 2 , it is possible to derive a calculation formula: r=(k×Q/π) 1/2 . 
     In a case that the volume of the ink droplet Id is V, since V is represented by: V=(4/3)×π×r 3 , it is possible to derive a calculation formula: V=(4/3)×π×(k×Q/π) 3/2  based on the above calculation formula for the “r”, and further to derive a calculation formula: V=(4/3)×π 1/3 ×(k×Q) 3/2 . 
     Therefore, as the calculation formula for calculating the volume of the ink droplet Id, it is possible to obtain a calculation formula: V=k1×Q 3/2 . The controller  71  calculates the volume V of the ink droplet Id by a calculation formula: V=k1×Q 3/2 . In other words, the volume of the ink droplet Id is proportional to Q 3/2 . Note that the “k1” is represented by k1=(4/3)×π 1/3 ×k 3/2 . Here, the controller  71  is capable of obtaining the time during which the current flows, based on the value of the current detected by the current detection part  81 . In this case, since Q=i×t is derived provided that the current value is i, the time during which the current flows is t, the controller  71  is capable of calculating the volume V of the ink droplet Id by a calculation formula: V=k1×(i×t) 3/2 . In such a manner, the controller  71  calculates the charge amount Q charged in one ink droplet Id, based on the product of the current value i detected by the current detection part  81  and the time t, to thereby make it possible to calculate the volume of the ink droplet Id. 
     As described above, the controller  71  calculates the volume of the ink droplet Id, and the controller  71  determines whether or not there is any discharge abnormality in the nozzle  21 , in accordance with the calculated volume of the ink droplet Id, periodically and before the printing. In this case, the controller  71  calculates the volume of the ink droplet Id in a case that the ink droplet Id is discharged, with respect to each of the nozzles  21 , and with respect to each of the nozzle arrays in the discharge head  20 . From the viewpoint of reducing the influence of noise, it is allowable that the ink droplet Id is discharged, for example, 10 times per each of the nozzles  21  so as to calculate the average value of the volumes of the ink droplet Id, and to make the determination regarding the presence or absence of discharge abnormality by comparing the average value with the threshold value. 
     In the present embodiment, the controller  71  determines whether or not there is a discharge abnormality of the nozzle  21  according to the calculated volume of the ink droplet Id; in a case that there is any discharge abnormality, the controller  71  changes the discharge waveform in a correction processing. In this case, the controller  71  determines whether or not to correct the volume of ink droplet Id based on a comparison between the reference value stored in the RAM  72  and the product of the current value i detected by the current detection part  81  and the time t during which the current flows (that is, the charge amount Q). 
     In the correction processing for changing the discharge waveform, a correction function in accordance with the calculated volume of the ink droplet Id is used. The correction function defines a voltage of the discharge pulse and a timing (positions at each of which the discharge pulse is made to be High or Low, with respect to each period of the time) of the discharge pulse in the discharge waveform. The controller  71  corrects the discharge waveform by using the above-described correction function based on the calculated volume of the ink droplet Id. Then, the controller  71  causes the waveform generation circuit  76  to generate a corrected discharge waveform based on the result of the calculation. Note that the above-described correction function is previously stored in the RAM  72  or the ROM  73 . 
     For example, it is allowable to perform the correction of the discharge waveform as follows. As depicted in  FIG.  7   , before the correction processing, the voltage of discharge pulses P1, P2, and P3 are same, whereas a pulse width and a discharge timing (time at which each of the discharge pulses P1, P2 and P3 is made to be High) are mutually different among the discharge pulses P1, P2 and P3. In a case that the discharge abnormality is recognized, a discharge waveform including discharge pulses P1a, P2a, and P3a is generated. The discharge pulse P1a has a lower voltage and a smaller pulse width than those of the discharge pulse P1, and has a discharge timing which is different from that of the discharge pulse P1. The discharge pulse P2a has a higher voltage and a larger pulse width than those of the discharge pulse P2, and has a discharge timing which is different from that of the discharge pulse P2. The discharge pulse P3a has a higher voltage and a larger pulse width than those of the discharge pulse P3, and has a discharge timing which is different from that of the discharge pulse P3. 
     As described above, according to the liquid droplet discharge device  10  of the present embodiment, the controller  71  calculates the volume of the ink droplet Id, based on the value of the current flowing between the electrode  84  and the discharge head  20  and detected by the current detection part  81 . With this, it is possible to determine, based on the volume of the ink droplet Id, whether or not the actuator  60 , which is the piezoelectric element, has deteriorated in the preceding stage before the nozzle reaches the non-discharge. Further, it is possible to perform the correction for adjusting the volume of the ink droplet Id, based on the result of the discrimination. As a result, it is possible to suppress a deterioration of the discharge accuracy of each of the nozzles  21  and to obtain a desired image quality. 
     Furthermore, in the present embodiment, the controller  71  calculates the volume of the ink droplet Id based on the product of the value of the current detected by the current detection part  81  and the time during which the current flows. In this case, the volume of the ink droplet Id can be easily calculated. 
     Moreover, in the present embodiment, the controller  71  calculates the charge amount charged on one ink droplet Id, from the current value detected by the current detection part  81 , and calculates the volume of the ink droplet Id based on the calculated charge amount. In this case, the volume of the ink droplet Id can be calculated almost accurately. 
     Further, in the present embodiment, under a condition that the ink droplet Id discharged from the discharge head  20  is separated from the nozzle surface  40   a , in a case that the distance d1 from the nozzle surface  40   a  to the tip position Ps in the discharge direction Dt of the ink droplet Id is less than the threshold value, the volume of the ink droplet Id is calculated by the calculation formula: V=k1×Q 3/2 . In this case, it is possible to accurately calculate the volume of the ink droplet Id, based on the state of the ink droplet Id at the time of discharge. 
     Furthermore, in the present embodiment, the electrode  84  is provided on the inside of the purge unit  83 . In this case, since it is not necessary to provide a new space at the outside of the purge unit  83 , the space can be saved. 
     Moreover, in the present embodiment, the controller  71  determines whether or not to correct the volume of the ink droplet Id, based on the comparison between the reference value and the product of the value of the current detected by the current detection part  81  and the time during which the current flows. By providing the reference for the comparison, it is possible to easily perform the determination as to whether or not to perform the correction. 
     Further, in the present embodiment, the controller  71  changes the discharge waveform in a case that the controller  71  performs the above-described correction. In this case, it is possible to correct the volume of the ink droplet Id by a simple method. 
     Second Embodiment 
     A second embodiment will be explained with reference to  FIGS.  8  and  9   . 
     As depicted in  FIG.  8   , there is such a case that, after an ink column Ide protrudes from the nozzle surface  40   a  and extends in the discharge direction Dt, a tip part of the ink column separates from the nozzle surface  40   a , as an ink droplet Id, due to the viscosity of the ink, etc. In this case, a distance d3 from the nozzle surface  40   a  to the tip position Ps of the ink column Ide in the discharge direction Dt in a case that the ink droplet Id separates from the nozzle surface  40   a  is not less than a threshold value. In this case, the volume of the ink droplet Id can be calculated as follows. Note that, in  FIG.  8   , a distance between the tip position Ps of the ink column Ide and the electrode  84  is defined as a distance d2, and a value obtained by subtracting the distance d2 from the distance d which is from the nozzle surface  40   a  to the electrode  84 , is defined as the distance d3. 
     As depicted in  FIG.  9   , it is possible to use a calculation formula for obtaining an electric field strength in a case that a voltage is applied between a needle electrode  110  having a needle tip  110   a  of a radius of curvature R and a flat plate electrode  111  having a gap length of “t” with respect to the needle electrode  110 . An electric field strength E of the needle tip  110   a  in a case that a voltage V2 is applied between the needle electrode  110  and the flat plate electrode  111  can be generally calculated by a calculation formula (hereinafter referred to as an approximate formula) represented by: E=2×V2/[2.3R×log (1+4t/R)]. By using this approximate formula, the volume of the ink droplet Id in the case of  FIG.  8    is obtained as follows. Note that log (x) is a logarithm of x with the base  10 , and that LN (x) (which will be described later) is a logarithm of x with the base “e”. 
     A cross-sectional area of the ink droplet Id is represented by S, 2.3 log (x)=LN (x) is true in the above-described approximate formula, the gap length t is replaced by the distance d2, and the radius of curvature R is replaced by the radius r of the ink droplet Id. Since the charge amount Q is proportional to E×S, a calculation formula: Q=k4×2×V2/[r×LN (1+4×d2/r)] can be derived. In the formula, “k4” is a predetermined coefficient. Here, since S=π×r 2 , a calculation formula: Q=k4×2×V2×π×r/LN (1+4×d2/r) can be derived. Note that, however, since the LN (x) does not change significantly even in a case that the radius “r” of the ink droplet Id changes, a calculation formula: Q is proportional to “r” can be derived. According to the above-described calculation formula of Q and the calculation formula of V=(4/3)×π×r 3 , the volume of the ink droplet Id can be regarded as: V=k2×Q 3 . Therefore, in a case that the distance d3 is not less than the threshold value, the controller  71  calculates the volume of the ink droplet Id by the calculation formula: V=k2×Q 3 . 
     According to the present embodiment, in the case that the distance d3 is not less than the threshold value, the controller  71  calculates the volume of the ink droplet Id by the calculation formula: V=k2×Q 3 . In this case, in a similar manner to the first embodiment, it is possible to accurately or appropriately calculate the volume of the ink droplet Id, in accordance with the state of the ink droplet Id at the time of discharge. 
     While the invention has been described in conjunction with various example structures outlined above and illustrated in the figures, various alternatives, modifications, variations, improvements, and/or substantial equivalents, whether known or that may be presently unforeseen, may become apparent to those having at least ordinary skill in the art. Accordingly, the example embodiments of the disclosure, as set forth above, are intended to be illustrative of the invention, and not limiting the invention. Various changes may be made without departing from the spirit and scope of the disclosure. Therefore, the disclosure is intended to embrace all known or later developed alternatives, modifications, variations, improvements, and/or substantial equivalents. Some specific examples of potential alternatives, modifications, or variations in the described invention are provided below. 
     (Modifications) 
     In the first embodiment, the volume of the ink droplet Id is calculated by the calculation formula: V=k1×Q 3/2 . In the second embodiment the volume of the ink droplet Id is calculated by the calculation formula: V=k2×Q 3 . In this regard, considering that the degree of change in the volume V of the ink droplet Id with respect to the different charge amounts Q in the first and second embodiments is approximately in a range of 0.5 times to 2.0 times, the relationship between the volume of the ink droplet Id and the charge amount Q can be regarded as linear. Therefore, the volume of the ink droplet Id can be represented, provided that k3 is a predetermined coefficient, by a calculation formula: V=k3×Q. Therefore, the controller  71  can calculate the volume of the ink droplet Id by the calculation formula: V=k3×Q. In this case, since the volume of the ink droplet Id and the charge amount are in a proportional relationship, the calculation of the volume becomes easy. Note that it is allowable to use a charge amplifier, rather than using the current detection part  81 , so as to construct a circuit in which the charge amount can be directly obtained by the charge amplifier. 
     Further, in the above-described embodiments, although the voltage, the pulse width, and the discharge timing are changed in the correction processing of the discharge waveform, the present disclosure is not limited to this. The content of the correction processing is merely an example; it is possible to appropriately set the combination of the voltage, the pulse width and the discharge timing which are to be changed before and after the correction processing. 
     Furthermore, the ink used in the above-described embodiments is not particularly limited, provided that the ink droplet Id is chargeable; it is possible to use, for example, a variety of kinds of the ink, such as dye ink, a pigment ink, etc.