Patent Publication Number: US-8113617-B2

Title: Ink-jet recording apparatus for current detection during startup and termination

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     The present application claims priority from Japanese Patent Application No. 2007-256198, which was filed on Sep. 28, 2007, the disclosure of which is herein incorporated by reference to its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates in general to an ink-jet recording apparatus which performs recording or printing by ejecting ink droplets. 
     2. Discussion of Related Art 
     An ink-jet head of an ink-jet printer for ejecting ink droplets onto a recording medium such as a recording sheet includes: a flow-passage unit in which are formed nozzles through which ink droplets are ejected and pressure chambers which communicate with the nozzles; actuators which apply an ejection energy to ink in the pressure chambers; and a driver IC in which are incorporated signal output circuits which output drive signals for driving the actuators. As the actuators each configured to apply a pressure to the ink in the pressure chambers by changing the volume thereof, there is known one disclosed in Patent Document 1 including: a piezoelectric sheet (piezoelectric layer) extending over a plurality of pressure chambers; a plurality of individual electrodes facing respectively the plurality of pressure chambers; and a common electrode (ground electrode) which faces the plurality of individual electrodes via the piezoelectric sheet and to which a base potential is given. In the disclosed actuators, when a drive pulse signal outputted from the signal output circuit of the driver IC is inputted to one of the individual electrodes, an electric field is generated at a portion of the piezoelectric sheet interposed between the above-indicated one individual electrode and the common electrode in a thickness direction of the piezoelectric sheet, so that the piezoelectric sheet at that portion expands or elongates in the thickness direction. Accordingly, the volume of the pressure chamber that corresponds to the one individual electrode is changed, whereby the pressure (ejection energy) is given to the ink in that pressure chamber. 
     In the ink-jet printer, there may be a risk that a short circuit occurs in the driver IC or the actuators due to breakage thereof, causing an overcurrent to flow in the signal output circuits. In particular when the ink-jet printer is in a non-operating state or in a standby state, the short circuit occurs at high frequency due to occurrence of migration phenomenon and entry of the ink from the pressure chambers into the corresponding actuators. When the ink-jet printer is turned on with the short circuit occurring, the overcurrent flows in the signal output circuits. To monitor the overcurrent, an electric current detecting circuit may be provided for detecting an electric current from the power supply device. However, since a maximum value of the electric current consumed when all of the actuators of the printer are placed in a charged state upon startup of the printer is high, it is difficult to detect the overcurrent when the overcurrent flows only in the signal output circuits corresponding to a part of the actuators. In view of this, there has been disclosed, in Patent Document 2, for instance, a technique of monitoring the electric current in each signal output circuit by disposing the electric current detecting circuit for each of the signal output circuits corresponding to the actuators.
     Patent Document 1 JP 2002-36568 (FIG. 1)   Patent Document 2 JP 2002-127405 (FIG. 1)   

     SUMMARY OF THE INVENTION 
     Where the electric current detecting circuit is disposed for each of the signal output circuits that correspond to the actuators as disclosed in the above-indicated Patent Document 2, the cost of manufacture of the ink-jet printer inevitably increases and the control circuit for the actuators tends to be large-sized. Further, when all of the actuators are simultaneously placed in the charged state upon startup of the ink-jet printer, all of the actuators are simultaneously displaced, thereby causing the ink to leak through the nozzles. In this instance, a large inrush current flows, whereby electromagnetic noise is generated and a heavy load is applied to the power supply device, resulting in misoperation of a protective circuit. 
     It is therefore an object of the present invention to provide an ink-jet recording apparatus which is capable of reducing a load to be imposed on a power supply device and inhibiting unnecessary behavior of actuators while inspecting the actuators and signal output circuits for a short circuit, upon startup of the apparatus, and which ensures a reduction in its size and a reduction in its manufacturing cost. 
     The above-indicated object may be attained according to a principle of the invention, which provides an ink-jet recording apparatus, comprising: 
     a flow-passage unit including a plurality of pressure chambers, a plurality of nozzles provided so as to respectively correspond to the plurality of pressure chambers, and a plurality of individual ink passages through which the plurality of pressure chambers respectively communicate with the plurality of nozzles; 
     a plurality of actuators including a plurality of individual electrodes provided so as to respectively correspond to the plurality of pressure chambers, a ground electrode which is disposed so as to face to the plurality of individual electrodes and to which is given a base potential, and a piezoelectric layer interposed between the plurality of individual electrodes and the ground electrode; 
     a plurality of signal output circuits which are provided so as to respectively correspond to the plurality of actuators and each of which outputs a signal for giving a drive potential to a corresponding one of the plurality of individual electrodes for placing a corresponding one of the plurality of actuators into a charged state and each of which outputs a signal for giving the base potential to the corresponding one of the plurality of individual electrodes for placing the corresponding one of the plurality of actuators into a discharged state; 
     at least one power supply device which supplies, to the plurality of signal output circuits, an electric power for giving the drive potential to the plurality of individual electrodes; 
     at least one electric current detecting device which detects an electric current with respect to the electric power supplied by the at least one power supply device; and 
     a control device including (a) a startup charging portion which controls the plurality of signal output circuits, upon startup of the ink-jet recording apparatus, such that all of the plurality of actuators are placed into the charged state from the discharged state in order at least one by one, when all of the plurality of actuators are in the discharged state and (b) a power-supply inhibiting portion which inhibits the electric power from being supplied from the at least one power supply device when the electric current detected by any of the at least one electric current detecting device is equal to or larger than a threshold during execution of a control by the startup charging portion. 
     In the ink-jet recording apparatus constructed as described above, the actuators are placed into the charged state from the discharged state at least one by one, upon startup of the apparatus, when the actuators are in the discharged state. Accordingly, a maximum value of the electric current consumed when the actuators are placed into the charged state can be lowered, thereby lowering the threshold used for the determination as to whether the electric power supply from the at least one power supply device is inhibited by the power-supply inhibiting portion, namely, as to the determination whether a short circuit is occurring. It is, therefore, possible to inspect the actuators and the signal output circuits for a short circuit without providing the electric current detecting device for each of the signal output circuits, ensuring a reduction in the size of the apparatus and a reduction in the cost of manufacture of the apparatus. Further, because the inrush current is suppressed, the load to be imposed on the at least one power supply device can be reduced and the unnecessary behavior of the actuators are inhibited for thereby preventing the ink from leaking from the nozzles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features, advantages and technical and industrial significance of the present invention will be better understood by reading the following detailed description of a preferred embodiment of the invention, when considered in connection with the accompanying drawings, in which: 
         FIG. 1  is an external side view showing an ink-jet printer according to one embodiment of the present invention; 
         FIG. 2  is a cross sectional view showing an ink-jet head of the ink-jet printer of  FIG. 1  taken along a width direction of the ink-jet head; 
         FIG. 3  is a plan view of a head body of the ink-jet head shown in  FIG. 2 ; 
         FIG. 4  is an enlarged view of a region enclosed by one-dot chain line in  FIG. 3 ; 
         FIG. 5  is a cross sectional view taken along line V-V in  FIG. 4 ; 
         FIG. 6A  is an enlarged cross sectional view of an actuator unit of  FIG. 4  and  FIG. 6B  is a plan view of an individual electrode disposed on a surface of the actuator unit; 
         FIG. 7  is a functional block diagram of a controller shown in  FIG. 1 ; 
         FIG. 8  is a circuit diagram of a signal output circuit of a driver IC shown in  FIG. 2 ; 
         FIG. 9  is a flow chart showing a procedure of startup processing upon startup of the ink-jet printer of  FIG. 1 ; and 
         FIG. 10  is a flow chart showing a procedure of termination processing upon terminating processing of the ink-jet printer of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     There will be described one embodiment of the invention with reference to the drawings. 
       FIG. 1  is a schematic side elevational view showing an overall structure of an ink-jet printer as an ink-jet recording apparatus, according to one embodiment of the present invention. The ink-jet printer generally indicated at  101  in  FIG. 1  is a color ink-jet printer having four ink-jet heads  1  and a controller  16  for controlling operations of the ink-jet printer  101 . In the ink-jet printer  101 , a sheet-supply portion  11  and a sheet-discharge portion  12  are disposed respectively at a left-side end portion and a right-side end portion in  FIG. 1 . 
     In the inside of the ink-jet printer  101 , there is formed a sheet-feed path through which a sheet (as a recording medium) P is fed in a sheet-feed direction from the sheet-supply portion  11  toward the sheet-discharge portion  12 . On a downstream side of the sheet-supply portion  11 , there is disposed a pair of sheet-feed rollers  5   a ,  5   b  by which the sheet P is fed while being held therebetween. The sheet-feed rollers  5   a ,  5   b  feed the sheet toward a rightward direction in  FIG. 1  from the sheet-supply portion  11 . At a middle portion in the sheet-feed path, there is disposed a feed mechanism  13  that includes two belt rollers  6 ,  7 , an endless feed belt  8  wound around the two belt rollers  6 ,  7  so as to be stretched therebetween, and a platen  15  disposed in a region enclosed by the feed belt  8 . The platen  15  is for supporting, at a position where the platen  15  faces the ink-jet heads  1 , the feed belt  8  such that the feed belt  8  does not deflect downward. Further, a nip roller  4  is disposed so as to face the belt roller  7  for pressing the sheet P fed from the sheet-supply portion  11  by the sheet-feed rollers  5   a ,  5   b , toward an outer circumferential surface  8   a  of the feed belt  8 . 
     The belt roller  6  is rotated by a feed motor (not shown) and the feed belt  8  is accordingly moved, so that the sheet P pressed by the nip roller  4  onto the outer circumferential surface  8   a  of the feed belt  8  is fed toward the sheet-discharge portion  12  while adhering to and being held by the feed belt  8 . The feed belt  8  has a silicone resin layer with low adhesion property formed on the outer circumferential surface  8   a  thereof. 
     On the downstream side of the feed belt  8  in the sheet-feed direction, there is provided a sheet separation mechanism  14  configured to separate the sheet P adhering to the outer circumferential surface  8   a  of the feed belt  8  and to guide the sheet P toward the sheet-discharge portion  12  disposed at the right-side end portion in  FIG. 1 . 
     The four ink-jet heads  1  respectively correspond to inks of four colors, i.e., magenta, yellow, cyan, and black, and are arranged in the sheet-feed direction. Accordingly, the ink-jet printer  101  is of a line type. Each of the four ink-jet heads  1  has a head body  2  at a lower end thereof. The head body  2  is a rectangular parallelepiped having a larger length in a direction perpendicular to the sheet-feed direction. The head boy  2  has a bottom surface functioning as an ink ejection surface  2   a  that faces the outer circumferential surface  8   a  of the feed belt  8 . When the sheet P fed on the feed belt  8  passes right below the four head bodies  2 , ink droplets of the four colors are ejected from the ink ejection surfaces  2   a  of the respective four head bodies  2  toward the upper surface or print surface (print region) of the sheet P. Thus, a desired color image is formed on the print region of the sheet P. 
     Referring next to  FIG. 2 , the ink-jet head  1  will be explained in detail.  FIG. 2  is a cross sectional view of the ink-jet head  1  taken along the width direction of the same  1 . As shown in  FIG. 2 , the ink-jet head  1  is constituted by a flow-passage section in which flow passages are formed, an electric-component section for permitting the ink droplets to be ejected from the flow-passage section, and a cover section for protecting the electric-component section. The flow-passage section includes: the head body  2  including a flow-passage unit  9  and actuator units  21 ; and a reservoir unit  71  disposed on the upper surface of the head body  2 . The reservoir unit  71  temporarily stores the ink to be supplied to the head body  2 . The electric-component section includes: a Chip On Film (COF)  50  on which driver ICs  52  are mounted; and a substrate  54  which is electrically connected to the COF  50 . The COF  50  is connected at its one end to the actuator units  21 , and a drive signal generated by each driver IC  52  is sent to the corresponding actuator unit  21 . The cover section is constituted by side covers  53  and a top cover  55 . The cover section accommodates the electric-component section therein for preventing entry of ink and mist of ink thereinto. 
     The reservoir unit  71  is formed by four plates  91 - 94  superposed on each other. In the reservoir unit  71 , there are formed an ink inlet passage (not shown), an ink reservoir  61 , and ten ink outlet passages  62 , which are in communication with each other. In  FIG. 2 , only one of the ten ink outlet passages  62  is shown. 
     The plate  94  has a recessed portion  94   a  formed in its surface facing the flow-passage unit  9 , so that a clearance is defined between the plate  94  and the flow-passage unit  9 . The actuator unit  21  is disposed in the clearance. The ink flowed into the ink reservoir  61  passes through the ink outlet passages  62  and is supplied to the flow-passage unit  9  via respective ink supply holes  105   b  of the flow-passage unit  9 . 
     The COF  50  is bonded, at the vicinity of its one end, to the upper surface of each actuator unit  21  for electrical connection with individual electrodes  135  and a common electrode  134  which will be explained. Further, the COF  50  is drawn upward from the upper surface of each actuator unit  21  so as to extend between one of the side covers  53  and the reservoir unit  71 , and is connected, at the other end thereof, to the substrate  54  via a connector  54   a . The substrate  54  is for relaying drive signals from the controller  16  to each driver IC  52 . 
     Referring next to  FIGS. 3-6 , the head body  2  will be explained.  FIG. 3  is a plan view of the head body  2  and  FIG. 4  is an enlarged view of a region enclosed by one-dot chain line in  FIG. 3 . In  FIG. 4 , pressure chambers  110 , apertures  112 , and nozzles  108  located below the actuator units  21  are indicated by solid lines instead of broken lines for convenience sake.  FIG. 5  is a partial cross sectional view taken along line V-V in  FIG. 4 .  FIG. 6A  is an enlarged cross sectional view of the actuator unit  21  and  FIG. 6B  is a plan view of the individual electrode  135  provided on the actuator unit  21 . 
     As shown in  FIG. 3 , the head body  2  is constituted by the flow-passage unit  9  and the four actuator units  21  fixed on an upper surface  9   a  of the flow-passage unit  9 . As shown in  FIG. 4 , each actuator unit  21  includes a plurality of actuators provided so as to face the respective pressure chambers  110  formed in the flow-passage unit  9  and has a function of selectively giving an ejection energy to the ink in the pressure chambers  110 . 
     The flow-passage unit  9  is a rectangular parallelepiped having substantially the same shape in plan view as the plate  94  of the reservoir unit  71 . The ten ink supply holes  105   b  are open to the upper surface  9   a  of the flow-passage unit  9  so as to respectively correspond to the ten ink outlet passages  62  ( FIG. 2 ) of the reservoir unit  71 . In the flow-passage unit  9 , there are formed: manifolds  105  communicating with the corresponding ink supply holes  105   b ; and sub manifolds  105   a  branched from the corresponding manifolds  105 . On a lower surface of the flow-passage unit  9 , the ink ejection surface  2   a  is formed in which a multiplicity of the nozzles  108  are arranged in a matrix, as shown in  FIGS. 4 and 5 . Like the nozzles  108 , the pressure chambers  110  are formed in a matrix on the upper surface  9   a  of the flow-passage unit  9  to which the actuator units  21  are fixed. 
     The flow-passage unit  9  is constituted by nine metal plates  122 - 130  each formed of stainless steel or the like and each having a rectangular shape in plan view that is elongate in a main scanning direction. 
     The plates  122 - 130  are superposed on each other while being aligned with each other, whereby through-holes formed in the respective plates  122 - 130  are connected to each other to form, in the flow-passage unit  9 , a multiplicity of individual ink passages  132  extending from the manifolds  105  to the nozzles  108  via the sub manifolds  105   a  and the pressure chambers  110 . 
     The ink supplied from the reservoir unit  71  into the flow-passage unit  9  flows into the individual ink passages  132  from the manifolds  105  (the sub manifolds  105   a ) and reaches the nozzles  108  via the apertures or orifices  112  and the pressure chambers  110 . 
     The actuator unit  21  will be explained. As shown in  FIG. 3 , the four actuator units  21  have a trapezoidal shape in plan view and are arranged in a zigzag fashion so as not to overlap the ink supply holes  105   b . Two parallel sides in each of the trapezoidal actuator units  21  extend in a longitudinal direction of the flow-passage unit  9  while two oblique sides of adjacent two actuator units  21  partially overlap each other with respect to the width direction of the flow-passage unit  9 , namely, in a sub scanning direction. 
     As shown in  FIG. 6A , each actuator unit  21  is constituted by three piezoelectric sheets or layers  141 - 143  each of which is formed of a ceramic material of lead zirconate titanate (PZT) having ferroelectricity. The individual electrodes  135  are formed at portions of the upper surface of the uppermost piezoelectric sheet  141  that correspond to the respective pressure chambers  110 . The common electrode (ground electrode)  134  is interposed between the uppermost piezoelectric sheet  141  and the piezoelectric sheet  142  located under the sheet  141 , so as to extend over entire surfaces of the sheets  141 ,  142 . As shown in  FIG. 6B , each individual electrode  135  has a generally rhombic shape in plan view similar to the pressure chambers  110 . One acute end of the individual electrode  135  is extended, and a circular conductive land  136  is provided at the extended end. 
     In each actuator unit  21 , the common electrode  134  is given a ground potential (base potential). The individual electrodes  135  are electrically connected to respective signal output circuits  52   a  as shown in  FIG. 8  provided in the driver IC  52 , via the corresponding lands  136  and the internal wiring of the COF  50 . In each actuator unit  21 , a portion sandwiched by and between each individual electrode  135  and a corresponding one of the pressure chambers  110  functions as one actuator. 
     The manner of driving or activating the actuator unit  21  will be explained. The piezoelectric sheet  141  is sandwiched by and between the multiplicity of individual electrodes  135  and the common electrode  134  while the piezoelectric sheets  142 ,  143  are sandwiched by and between the common electrode  134  and the upper surface  9   a  of the flow-passage unit  9 . A portion of the piezoelectric sheet  141  sandwiched by and between each individual electrode  135  and the common electrode  134  functions as an active layer which is configured to contract or expand in a direction parallel to the surface of the sheet  141  (hereinafter referred to as “surface direction”) upon application of a voltage between the electrodes  135 ,  134 . The portion functioning as the active layer deforms so as to change the volume of the corresponding pressure chamber  110 , cooperating with the piezoelectric sheets  142 ,  143  that are nearer to the pressure chambers  110  than the sheet  141 . Where the polarization direction of the active layer and the direction of the electric field are both in the thickness direction of the sheets  141 - 143 , the active layer contracts in the surface direction, and a portion of the sheets  141 - 143  corresponding to the individual electrode  135  deforms convexly toward the corresponding pressure chamber  110  (i.e., unimorph deformation). Accordingly, a pressure (ejection energy) is given to the ink in the pressure chamber  110  and therefore a pressure wave is generated in the same  110 . The generated pressure wave propagates from the pressure chamber  110  to the corresponding nozzle  108 , so that the ink droplet is ejected from the nozzle  108 . 
     In the present embodiment, each actuator is, in advance, kept in a charged state by applying a drive potential to the corresponding individual electrode  135 , and the volume of the corresponding pressure chamber  110  is decreased. Each time when an ejection requirement is made, there is outputted from the driver IC  52  a drive signal for once placing the actuator into the discharged state by applying the ground potential to the corresponding individual electrode  135  and again placing the actuator into the charged state by again applying the drive potential to the corresponding individual electrode  135  at suitable timing. In this instance, at timing when the potential of the individual electrode  135  becomes equal to the ground potential, the pressure of the ink in the corresponding pressure chamber  110  decreases, namely, the volume of the pressure chamber  110  increases, so that the ink is sucked from the corresponding sub manifold  105   a  into the corresponding individual ink passage  132 . Subsequently, at timing when the potential of the individual electrode  135  becomes again equal to the drive potential, the pressure in the pressure chamber  110  increases, namely, the volume of the pressure chamber  110  decreases, so that the ink droplet is ejected from the corresponding nozzle  108 . That is, a pulse with a rectangular waveform is applied to the individual electrode  135 . The width of the pulse is equal to an acoustic length AL which is a time length required for the pressure wave to propagate from the outlet of the sub manifold  105   a  to the leading end of the nozzle  108  through the pressure chamber  110 . When the pressure of the ink in the pressure chamber  110  changes from the negative pressure state to the positive pressure state, the ink droplet can be ejected from the nozzle  108  by a strong pressure because the pressure generated upon the volume decrease is added. In the present embodiment, the drive potential is equal to 24V. 
     In the ink ejecting operation by the actuator unit  21 , when the potential of the individual electrode  135  is changed from the ground potential to the drive potential (e.g., 24V) and vice versa, a transient current flows. When the potential of the individual electrode  135  is changed from the ground potential to the drive potential, the transient current is supplied by a control power supply device  85 . When the potential of the individual electrode  135  is changed from the ground potential to the drive potential, the charging current flows to the individual electrode  135 . When the potential of the individual electrode  135  is changed from the drive potential to the ground potential, the discharging current flows from the individual electrode  135 . Where there are no deficiencies in the actuator units  21  and the driver ICs  52  (the signal output circuits  52   a ), a prescribed transient current flows each time when the potential of the individual electrode  135  changes. In the present embodiment, the transient current flows for about 1 μsec immediately after the voltage transition. 
     Referring next to the functional block diagram of  FIG. 7 , the controller  16  will be explained. As shown in  FIG. 7 , the controller  16  includes: an image-data storing portion  86 ; a head control portion  87  as a control device; a head power supply device  88  as a power supply device; and an electric current detecting circuit  89  as an electric current detecting device. The head control portion  87 , the head power supply device  88 , and the electric current detecting circuit  89  are provided on each of the four ink-jet heads  1 . In  FIG. 7 , there are shown only one ink-jet head  1 , and the head control portion  87 , the head power supply device  88  and the electric current detecting device  89  which are connected to the above-indicated one ink-jet head  1 . The controller  16  works by an electric power of 3.3V system supplied thereto from the control power supply device  85 . 
     The image-data storing portion  86  stores image data of an image to be printed on the sheet P. The image data includes dot data of each of dots of the image to be printed. In the present embodiment, the dot data is constituted such that the volume of the ink droplet to be ejected from each nozzle  108  is represented in four tones. 
     The head control portion  87  controls activation of the actuator units  21  and includes a recording control portion  75 , a startup charging portion  76 , a termination discharging portion  77 , and a power-supply inhibiting portion  78 . The recording control portion  75  controls activation of each actuator unit  21  via the corresponding driver IC  52  such that the ink droplets are ejected from the nozzles  108  at desired timing according to the image data stored in the image-data storing portion  86 . Each driver IC  52  includes a plurality of signal output circuits  52   a  for driving or activating respective actuators which are formed in the corresponding actuator unit  21  and which correspond to the respective nozzles  108 . Hereinafter, each actuator is referred to as “a channel” where appropriate. 
     Referring next to the circuit diagram of  FIG. 8 , the operation of each signal output circuit  52   a  of the driver IC  52  will be explained. As shown in  FIG. 8 , the signal output circuit  52   a  includes: a p-channel transistor (MOS FET) TR 1 ; an n-channel transistor (MOS FET) TR 2 ; protective diodes D 1 , D 2  each of which is disposed between a drain and a source of a corresponding one of the transistor TR 1  and the transistor TR 2 ; and a drive resistor R 3 . The drain of the transistor TR 1  and the source of the transistor TR 2  are connected. The output terminal of the head control portion  87  is connected to gates of the respective transistors TR 1 , TR 2 , and a control signal from the head control portion  87  is inputted to the transistors TR 1 , TR 2 . The drive resistor R 3  is disposed between the individual electrode  135  and a connection of the two transistors TR 1 , TR 2 . The electric current value to be supplied to the individual electrode  135  is determined by the drive resistor R 3 . Where the control signal is at a Low level, the transistor TR 1  turns on while the transistor TR 2  turns off. In this state, the potential of 24V is given to the individual electrode  135  via the drive resistor R 3 , whereby the corresponding channel is charged. Where the control signal is at a High level, on the other hand, the transistor TR 1  turns off while the transistor TR 2  turns on. In this state, the ground potential is given to the individual electrode  135  via the drive resistor R 3 , whereby the corresponding channel is discharged. Thus, the signal output circuit  52   a  is an inverter circuit configured to give, to the corresponding individual electrode  135 , drive signals of 24V system logically inverted with respect to the control signal of 3.3V system from the head control portion  87 . 
     When the control signal from the head control portion  87  changes from the High level to the Low level or vice versa, both of the transistors TR 1 , TR 2  simultaneously turn on for a moment, and a through current flows through the both of the transistors TR 1 , TR 2 . To prevent the through current, a through-current preventive circuit that adjusts the transition timing of the signal level may be provided between the head control portion  87  and the gates of the respective transistors TR 1 , TR 2 . 
     Referring back to  FIG. 7 , the startup charging portion  76  is for controlling charging and discharging of all actuator units  21  of the ink-jet head  1  upon startup of the ink-jet printer  101 . Described more specifically, upon startup of the ink-jet printer  101 , after the controller  16  has been operated by the electric power of 3.3V system supplied from the control power supply device  85 , the start-up charging portion  76  outputs the control signal at the High level to the gates of the transistors TR 1 , TR 2  of the signal output circuits  52   a  corresponding to all of the channels of all of the actuator units  21 . In this state, the head power supply device  88  is allowed to output the electric power of 24V system. Accordingly, the signal for giving the ground potential to each individual electrode  135  is outputted from each of the signal output circuits  52   a  of each driver IC  52 , and all of the channels of all of the actuator units  21  are placed into the discharging state. Thereafter, for placing the actuator units  21  into the charged state from the discharged state in order one by one, the startup charging portion  76  controls each of the signal output circuits  52   a  of the corresponding driver IC  52  to output the signal for giving the drive potential to each individual electrode  135 . As a result, all of the channels of the all of the actuator units  21  are placed into the charged state, and the ink-jet printer  101  is placed into a standby mode. 
     The termination discharging portion  77  is for controlling charging and discharging of all actuator units  21  of the ink-jet head  1  upon terminating processing (termination) of the ink-jet printer  101 . More specifically explained, at the time of initiation of the termination processing, all of the channels of all of the actuator units  21  are kept placed in the charged state. Accordingly, on the precondition that all of the channels of all of the actuator units  21  are in the charged state upon initiation of the termination processing, the termination discharging portion  77  controls each of the signal output circuits  52  of each driver IC  52  to output the signal for giving the drive potential to each individual electrode  135 . When all of the channels of all of the actuator units  21  are in the charged state, for placing the actuator units  21  into the discharged state from the charged state in order one by one, the termination discharging portion  77  controls each of the signal output circuits  52   a  of the corresponding driver IC  52  to output the signal for giving the ground potential to each individual electrode  135 . More specifically explained, for placing the channels in each actuator unit  21  into the discharged state from the charged state in order one channel by one channel, the termination discharging portion  77  controls each of the signal output circuits  52   a  of the corresponding driver IC  52  to output the signal for giving the ground potential to each individual electrode  135 . Thereafter, the electric power supply of 24V system from the head power supply device  88  is stopped. Where the ink-jet printer  101  is configured such that the ink-jet printer  101  is kept in the standby mode with the channels of each actuator unit  21  placed in the discharged state, the termination discharging portion  77  may be configured to control, in the control for charging and discharging of the actuator units  21 , the signal output circuits  52   a  such that all channels are once placed into the charged state upon initiation of the terminating processing. 
     The head power supply device  88  is a 24V-system power source for driving the actuator units  21 . The electric power of 24V system outputted from the head power supply device  88  is supplied to the driver ICs  52  via the head control portion  87 . The electric current detecting circuit  89  is for detecting an electric current at the output portion of the head power supply device  88 . The result of detection by the detecting circuit  89  is sent to the power-supply inhibiting portion  78  of the head control portion  87 . 
     The electric current detecting circuit  89  detects the electric current except for a time period during which the transient current flows. The electric current detecting circuit  89  starts to detect the electric current in 1 μsec or longer after the potential transition. Accordingly, the electric current detected upon charging by the startup charging portion or upon discharging by the termination discharging portion  77  does not include the transient current. 
     When the startup charging portion  76  is performing the charging of the channels of each actuator unit  21  or when the termination discharging portion  77  is performing the discharging of the channels of each actuator unit  21 , the power-supply inhibiting portion  78  inhibits or stops the electric power of the 24V system from outputting from the head power supply device  88  where the electric current detecting circuit  89  detects an overcurrent, namely, an electric current equal to or larger than a prescribed threshold. By inhibiting the electric power supply of 24V system from the head power supply device  88 , the output of the drive signal from each signal output circuit  52   a  is stopped. 
     Referring next to  FIG. 9 , there will be explained startup processing upon startup of the ink-jet printer  101 . FIG.  9  is a flow chart showing a procedure of the startup processing for the ink-jet printer  101 . When a startup command for starting the ink-jet printer  101  is inputted by a user, step S 101  (hereinafter “step” is omitted where appropriate) is implemented to permit the control power supply device  85  to output the electric power of 3.3V system, so that the controller  16  is operated. S 101  is followed by S 102  in which the startup charging portion  76  outputs the control signal at the High level to the gates of the respective transistors TR 1 , TR 2  of the signal output circuits  52   a  corresponding to all channels of all actuator units  21 . 
     S 102  is followed by S 103  in which the head power supply device  88  outputs the electric power of 24V system. Because the control signal at High level has been inputted to the gates of the respective transistors TR 1 , TR 2  of each of the signal output circuits  52   a , each of the signal output circuits  52   a  of each driver IC  52  outputs the drive signal for giving the ground potential to the corresponding individual electrode  135 . As a result, all of the channels of all of the actuator units  21  are placed into the discharged state. 
     Thereafter, S 104  is implemented in which, for placing all channels of one of the actuator units  21  into the charged state from the discharged state, the startup charging portion  76  controls each of the signal output circuits  52   a  of the corresponding driver IC  52  to output the drive signal for giving the drive potential to the corresponding individual electrode  135 . S 104  is followed by S 105  to judge whether the electric current detecting circuit  89  has detected an overcurrent (short circuit). Where the short circuit is occurring between the individual electrode  135  and the common electrode  134  in one of the channels, for instance, the electric current flows in the signal output circuit  52   a  corresponding to that one channel via the shorted portion even after that channel has been placed into the charged state. In consequence, the electric current detecting circuit  89  detects an abnormal overcurrent. As a modification of the present embodiment, the drive signal for giving the drive potential to the individual electrode  135  may be outputted from each of the signal output circuits  52   a  of each driver IC  52  for placing the channels in the corresponding actuator unit  21  into the charged state from the discharged state in order one channel by one channel. In this instance, it is possible to detect a short circuit that is occurring between the two individual electrodes  135  of adjacent two channels, for instance. More specifically explained, when one of the two adjacent channels is kept in the charged state while the other is placed into the discharged state, the electric current flows from the signal output circuit  52   a  corresponding to the channel in the charged state to the signal output circuit  52   a  corresponding to the channel in the discharged state via the shorted portion. In such an instance, too, the electric current detecting circuit  89  detects an abnormal overcurrent. 
     Where it is judged in S 105  that the electric current detecting circuit  89  has detected an overcurrent (S 105 : YES), S 106  is implemented in which the power-supply inhibiting portion  78  inhibits or stops the electric power of 24V system from being outputted from the head power supply device  88 , and the occurrence of abnormality is indicated on a display not shown. Thus, one execution of the routine of the flow chart of  FIG. 9  is ended. On the other hand, where it is judged in S 105  that the electric current detecting circuit  89  has not detected any overcurrent (S 105 : NO), S 107  is implemented in which the startup charging portion  76  judges whether there exists another actuator unit  21  to be subsequently placed into the charged state. Where the startup charging portion  76  judges that there exists another actuator unit  21  to be subsequently placed into the charged state (S 107 : YES), the control flow goes back to S 104  and the above-indicated processing is repeated. On the other hand, where the startup charging portion  76  judges that there exists no actuator unit  21  to be subsequently placed into the charged state (S 107 : NO), one execution of the routine of the flow chart of  FIG. 9  is ended. When the startup processing by the startup charging portion  76  is completed, all of the channels of all of the actuator units  21  are kept in the charged state, so that the channels can be quickly driven for ejection of the ink droplets. 
     Referring next to  FIG. 10 , there will be explained terminating processing upon termination of the ink-jet printer  101 .  FIG. 10  is a flow chart showing a procedure of the terminating processing for the ink-jet printer  101 . It is noted that all of the channels of all of the actuator units  21  are kept placed in the charged state when the ink-jet printer  101  is in the standby mode for printing. In this state, when a termination command for terminating the ink-jet printer  101  is inputted by the user, step S 201  is implemented in which, for placing the channels of one of the actuator units  21  into the discharged state from the charged state in order one channel by one channel, the termination discharging portion  77  controls each of the signal output circuits  52   a  of the corresponding driver IC  52  to output the drive signal for giving the ground potential to the corresponding individual electrode  135 . As explained above, when one of the two adjacent channels is kept in the charged state while the other is placed into the discharged state in an instance where the short circuit is occurring between the two individual electrodes  135  of the adjacent two channels, the electric current flows from the signal output circuit  52   a  corresponding to the channel in the charged state to the signal output circuit  52   a  corresponding to the channel in the discharged state via the shorted portion, so that the electric current detecting circuit  89  detects an abnormal overcurrent. 
     S 201  is followed by S 202  in which it is judged whether the electric current detecting circuit  89  has detected an overcurrent (short circuit). Where it is judged in S 201  that the electric current detecting circuit  89  has detected an overcurrent (S 202 : YES), S 203  is implemented in which the occurrence of abnormality is indicated on the display. The control flow then goes to S 205  in which the power-supply inhibiting portion  78  inhibits or stops the output of the electric power of 24V system from the head power supply device  88  and subsequently to S 206  in which the termination discharging portion  77  stops the output of the electric power of 3.3V system from the control power supply device  85 . On the other hand, where it is judged in S 202  that the electric current detecting circuit  89  has not detected any overcurrent (S 202 : NO), S 204  is implemented in which the termination discharging portion  77  judges whether there exists another actuator unit  21  to be subsequently placed into the discharged state. Where the termination discharging portion  77  judges that there exists another actuator unit  21  to be subsequently placed into the charged state (S 204 : YES), the control flow goes back to S 201  and the above-indicated processing is repeated. On the other hand, where the termination discharging portion  77  judges that there exists no actuator unit  21  to be subsequently placed into the discharged state (S 204 : NO), S 205  is implemented in which the power-supply inhibiting portion  78  inhibits or stops the output of the electric power of 24V from the head power supply device  88 . S 205  is followed by S 206  in which the termination discharging portion  77  stops the output of the electric power of 3.3V system from the control power supply device  85 . Thus, the ink-jet printer  101  is terminated and one execution of the routine of the flow chart of  FIG. 10  is ended. 
     In the illustrated embodiment, the startup charging portion  76  places, upon startup of the ink-jet printer  101 , the channels in each of the actuator units  21  into the charged state from the discharged state, in order one actuator unit by one actuator unit. Accordingly, the inrush current upon charging of the actuators is small, whereby the electric current can be detected with high stability. Further, the electric current that flows when the short circuit occurs can be suppressed, ensuring accurate measurement of the electric current and reliable judgment as to whether the short circuit is occurring or not. Moreover, the maximum value of the electric current consumed upon charging is lowered, thereby lowering the threshold used for the determination as to whether the electric power supply from the head power supply device  88  is inhibited by the power-supply inhibiting portion  78 , namely, the determination as to whether the short circuit is occurring or not. It is, therefore, possible to inspect the actuator units  21  and the signal output circuits  52   a  for the short circuit, without providing the electric current detecting device for each of the signal output circuits  52   a , ensuring a reduction in the size of the ink-jet printer  101  and a reduction in the cost of manufacture of the same  101 . Further, because the inrush current (the transient current) can be suppressed, the load to be imposed on the head power supply device  88  can be reduced and the unnecessary behavior of the channels are inhibited for thereby preventing the ink leakage from the nozzles. 
     In the illustrated embodiment, the termination discharging portion  77  places, upon termination of the ink-jet printer  101 , each of the channels in each of the actuator units  21  into the discharged state from the charged state, in order one channel by one channel in each actuator unit  21 . It is, therefore, possible to inspect the actuator units  21  and the signal output circuits  52   a  for the short circuit even upon termination of the ink-jet printer  101 . Accordingly, when the short circuit is detected, appropriate measures can be taken before next startup of the ink-jet printer  101 . 
     In the illustrated embodiment, the head power supply device  88  is provided for each of the four ink-jet heads  1 . Accordingly, the maximum value of the electric current consumed upon startup of the ink-jet printer  101  can be further lowered, and the inrush current can be suppressed with higher reliability. 
     In the illustrated embodiment, the plurality of actuators are unified to form one actuator unit  21 , and the plurality of signal output circuits  52   a  corresponding to the plurality of actuators in one actuator unit  21  are included in one driver IC  52 . Each of the four ink-jet heads  1  includes the plurality of actuator units and the plurality of driver ICs  52  that correspond respectively to the actuator units  21 . Further, one head power supply device  88  is configured to supply the electric power to the signal output circuits  52   a  of each of the driver ICs  52  in one ink-jet head  1 . Because the number of the head power supply device  88  provided in one ink-jet head  1  is less than the number of the driver ICs  52  in one ink-jet head  1 , it is not necessary to provide the power supply device  88  and the electric current detecting circuit  89  individually for the respective driver ICs  52 . Accordingly, it is possible to identify a defective driver IC or ICs  52  by the electric current detecting circuit  89  whose number is smaller than that of the driver ICs  52 , contributing to a reduced cost of manufacture of the ink-jet printer  101 . 
     While the preferred embodiment of the invention has been described by reference to the accompanying drawings, for illustrative purpose only, it is to be understood that the present invention is not limited to the details of the illustrated embodiment, but may be embodied with various changes and modifications, which may occur to those skilled in the art, without departing from the spirit and scope of the invention defined in the attached claims. 
     As described above, in the illustrated embodiment, the startup charging portion  76  places, upon startup of the ink-jet printer  101 , the channels in each of the actuator units  21  into the charged state from the discharged state, in order one actuator unit by one actuator unit. The number of a set of the channels to be placed into the charged state is not limited to the number of the channels in each actuator unit  21 , but may be arbitrarily determined. For instance, the number of a set of the channels to be placed into the charged state may be equal to a number of the nozzles in one nozzle row formed in the ink ejection surface  2   a  or a total number of the nozzles in a plurality of nozzle rows for one sub manifold  105   a . Alternatively, the channels may be placed into the charged state one by one. In this instance, it is possible to find or identify a defective channel with high reliability. 
     As described above, in the illustrated embodiment, the termination discharging portion  77  places, upon termination of the ink-jet printer  101 , each of the channels in each of the actuator units  21  into the discharged state from the charged state, in order one channel by one channel in each actuator unit  21 . The number of a set of the channels to be placed into the discharged state is not limited to the number of the channels in each actuator unit  21 , but may be arbitrarily determined. For instance, the number of a set of the channels to be placed into the discharged state may be equal to the number of the nozzles in one nozzle row formed in the ink ejection surface  2   a  or the total number of the nozzles in a plurality of nozzle rows for one sub manifold  105   a . In the illustrated embodiment, the channels in each actuator unit  21  is placed into the discharged state one by one. In this instance, it is possible to find or identify a defective channel with high reliability. In this respect, the controller  16  may be configured such that the controller  16  does not have the termination discharging portion  77  and such that all of the channels are simultaneously placed into the discharged state from the charged state upon termination of the ink-jet printer  101 . 
     In the illustrated embodiment, the control by the startup charging portion  76  and the control by the termination discharging portion  77  are executed for each of the actuator units  21 , for detecting the electric current. Those controls may be executed for each of the driver ICs  52 . In this instance, even where one driver IC  52  outputs the drive signals to a plurality of actuator units  21 , it is possible to quickly confirm the presence or absence of any deficiency without obtaining the result of detection of the electric current and the judgment based on the detection result in the corresponding actuator units  21 . 
     In the illustrated embodiment, the head power supply device  88  is provided for each of the four ink-jet heads  1 . The head power supply device  88  may be provided for each of groups of ink-jet heads  1 .