Abstract:
There is disclosed a head driving device which drives a plurality of pressure generating elements for generating pressure fluctuation in a jetted object contained in each of associated pressure chambers formed in a jetting head of a jetting apparatus to eject the jetted object from each of nozzles communicated with the associated pressure chambers. In the device, a head driver generates a drive signal which is selectively applied to at least one of the pressure generating elements to be driven. A bias potential provider selectively applies a bias potential to at least one of the pressure generating elements not to be driven.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This is a continuation of application Ser. No. 10/854,783 filed May 27, 2004, which is a continuation of application Ser. No. 10/356,740 filed Feb. 3, 2003, which is U.S. Pat. No. 6,752,482 issued Jun. 22, 2004; the disclosures of which are incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The invention relates to a jetting head capable of ejecting various kinds of liquid in the form of droplets for use in an ink jet printer, a display manufacturing apparatus, an electrode forming apparatus, a biochip manufacturing apparatus, etc., and more particularly, to a jetting apparatus having a plurality of flexible flat cables to be used for supplying drive signals from a head driver to a jetting head.  
         [0003]     As a jetting apparatus having a jetting head capable of ejecting liquid in the form of a liquid droplet, for example, there has been proposed an ink jet printer in which ink droplets are ejected to record an image or the like on recording paper, an electrode forming apparatus in which an electrode material in a liquid form is ejected onto a substrate to thereby form electrodes, a biochip manufacturing apparatus in which biological samples are ejected to manufacture biochips, or a micropipette for ejecting a predetermined amount of a sample into a vessel.  
         [0004]     For instance, in an ink jet printer employing piezoelectric elements as drive elements for ejecting ink, a plurality of piezoelectric elements, which are provided so as to correspond to a plurality of nozzles of a print head, are selectively activated, whereby ink droplets are ejected from the nozzles in accordance with the dynamic pressure generated by the respective piezoelectric elements. Dots are formed on recording paper by causing the ink droplets to adhere to the recording paper, thus effecting printing operation.  
         [0005]     Here, the piezoelectric elements are provided so as to correspond to nozzles to be used for ejecting ink droplets. The piezoelectric elements are actuated by a drive signal supplied from a head driver mounted in the print head, thereby ejecting ink droplets.  
       SUMMARY OF THE INVENTION  
       [0006]     It is therefore an object of the invention to provide a device and a method for driving a jetting head designed to readily retain predetermined bias voltages of respective piezoelectric elements through use of a simple, compact configuration and at low cost.  
         [0007]     In order to achieve the above object, according to the invention, there is provided a head driving device, which drives a plurality of pressure generating elements for generating pressure fluctuation in a jetted object contained in each of associated pressure chambers formed in a jetting head of a jetting apparatus to eject the jetted object from each of nozzles communicated with the associated pressure chambers, the head driving device comprising:  
         [0008]     a head driver, which generates a drive signal which is selectively applied to at least one of the pressure generating elements to be driven; and  
         [0009]     a bias potential provider, which selectively applies a bias potential to at least one of the pressure generating elements not to be driven.  
         [0010]     In such a configuration, the non-actuated pressure generating elements are held at the bias potential. Accordingly, the voltage applied to both electrodes of the non-actuated pressure generating elements becomes substantially zero. Hence, power draw is reduced, and a voltage drop stemming from spontaneous discharge of the pressure generating elements becomes smaller. Hence, a power loss is diminished.  
         [0011]     Further, occurrence of discharge due to a potential difference between pressure generating elements to be driven and pressure generating elements not to be driven is also reduced. In addition, a further increase in arrangement density of a head can be attained without involvement of an operation for providing insulation between the electrodes of the pressure generating elements.  
         [0012]     Preferably, the bias potential is a reference potential of the drive signal.  
         [0013]     Preferably, the bias potential provider includes a potential applier which applies the bias potential, and a charger which charges the potential applier with a drive potential of the drive signal.  
         [0014]     Here, it is preferable that the charger includes a transistor which applies the drive potential to the potential applier, and a switcher which supplies the drive signal to a base terminal of the transistor during a time period in which the drive signal deactivates the pressure generating elements.  
         [0015]     In such a configuration, the transistor is turned on by the supplied drive signal to charge the potential applier with the bias potential.  
         [0016]     Here, it is further preferable that the switcher continuously supplies the drive signal before and after a Jetting operation is performed.  
         [0017]     Specifically, the drive signal is supplied to discharge the potential applier after the jetting operation is performed.  
         [0018]     Before the jetting operation, since the potential applier is gradually charged to reach the bias potential by the continuous supply of the drive signal, there is prevented occurrence of faulty operations of respective pressure generating elements, which would otherwise be caused by a sudden increase in the potential of the ground-side electrodes before commencement of the jetting operation.  
         [0019]     After the jetting operation, since the potential applier is gradually discharged by the continuous supply of the drive signal, there is prevented occurrence of faulty operations of the respective pressure generating elements, which would otherwise be caused by a sudden drop in the voltage of the ground-side electrodes after completion of the jetting operation.  
         [0020]     Further, it is preferable that: the head driver is mounted on the jetting head; and the switcher is embodied by a part of a switching circuit included in the head driver which selectively applies the drive signal to the at least one pressure generating elements to be driven.  
         [0021]     In such a configuration, the switcher is provided by utilizing a surplus unused section of an existing switching circuit of the head driver mounted on a jetting head, thereby curtailing the cost of parts. Further, a space to be used for mounting the switcher is not particularly required, thus rendering the apparatus compact.  
         [0022]     According to the invention, there is also provided a method of driving a jetting head provided with pressure generating elements, the method comprising steps of:  
         [0023]     generating a drive signal selectively applied to at least one of the pressure generating elements to be driven to eject jetted objects; and  
         [0024]     applying a bias potential from a potential applier to at least one of the pressure generating elements not to be driven.  
         [0025]     Preferably, the driving method further comprises a step of charging the potential applier with a drive potential of the drive signal.  
         [0026]     Here, it is preferable that the charging step is performed during a time period in which the drive signal deactivates the pressure generating elements.  
         [0027]     It is further preferable that the charging step is performed during a time period in which the drive signal deactivates the pressure generating elements.  
         [0028]     According to the invention, there is also provided a head driving device, which drives a plurality of pressure generating elements for generating pressure fluctuation in a jetted object contained in each of associated pressure chambers formed in a jetting head of a jetting apparatus to eject the jetted object from each of nozzles communicated with the associated pressure chambers, the head driving device comprising:  
         [0029]     a head driver, which generates a drive signal which is selectively applied to at least one of the pressure generating elements to be driven;  
         [0030]     a bias potential provider, which applies a bias potential to respective ground-side electrodes of the pressure generating elements; and  
         [0031]     an IC package, in which the head driver and the bias potential provider are provided.  
         [0032]     In such a configuration, the ground-side electrodes of the pressure generating elements are held at the bias potential.  
         [0033]     Accordingly, the voltage to be applied across both electrodes of the pressure generating elements is reduced. Therefore, power consumption is diminished, and a voltage drop stemming from spontaneous discharge of the pressure generating elements is small, thereby reducing a power loss.  
         [0034]     Further, since the voltage to be applied to the pressure generating elements becomes relatively low, electric discharge stemming from a voltage difference between pressure generating elements to be driven and pressure generating elements not to be driven is also reduced. In addition, a further increase in arrangement density of the pressure generating elements can be attained without involvement of an operation for providing insulation between the electrodes of the pressure generating elements, even when pressure generating elements eventually assume a lower withstand voltage.  
         [0035]     Since the head driver and the bias potential provider are provided integrally within an IC package, a reduction in packing, wiring, and connection space can be attained.  
         [0036]     Preferably, the bias potential is a reference potential of the drive signal.  
         [0037]     In such a configuration, the voltage applied to across electrodes of the pressure generating elements becomes substantially zero. Hence, a voltage drop stemming from spontaneous discharge of the pressure generating elements becomes smaller, thereby reducing a power loss.  
         [0038]     Preferably, the head driving device further comprising:  
         [0039]     a capacitor, having a capacitance which is sufficiently greater than a total electrostatic capacitance of the pressure generating elements, the capacitor provided with a first terminal which is electrically connected to the ground-side electrodes and a second terminal which is grounded; and  
         [0040]     a control resistor, which electrically connects the first terminal of the capacitor and the bias potential provider.  
         [0041]     In such a configuration, the capacitor is charged with a bias potential output from the bias potential provider by way of the control resistor. In a case where an amplifier is provided in the bias potential provider, since the charging voltage of the capacitor is applied to the pressure generating elements, it is not necessary to provide an amplifier of a high speed operable type. A low-speed, small-capacity amplifier can be used, thereby curtailing cost of such an amplifier.  
         [0042]     Due to the existence of the control resistor, the charging and discharged currents substantially do not flow into the amplifier of the bias potential provider, but flow into the condenser. Hence, the amount of heat dissipated by the amplifier is reduced.  
         [0043]     Here, it is preferable that the bias potential provider charges the capacitor with a potential according to a data signal inputted to the bias potential provider, so that the charged potential is applied to the ground-side electrodes of the pressure generating elements as the bias potential.  
         [0044]     Further, it is preferable that the bias potential provider discharges the capacitor according to a data signal inputted to the bias potential provider, so that the ground-side electrodes of the pressure generating elements are discharged.  
         [0045]     In such a configuration, due to the existence of the control resistor, a large discharged electric current does not flow into the bias potential provider, thereby lowering the amount of heat dissipated by e.g., an amplifier of the bias potential provider.  
         [0046]     Further, it is preferable that the data signal is inputted to the head driver to generate the drive signal.  
         [0047]     In such a configuration, a data signal can be input from a common connection terminal of an IC package constituting the head driver and the bias potential provider. Accordingly, inputting a data signal individually to the head driver and to the bias potential provider is not required, thereby reducing the wiring and connection space.  
         [0048]     Further, it is preferable that the head driving device further comprises a temperature detector, which detects a temperature of the jetting head. The data signal corresponds to the bias potential which is determined by the detected temperature.  
         [0049]     Alternatively, it is preferable that the number of bits forming the data signal is less than the number of a signal inputted to the head driver to generate the drive signal.  
         [0050]     The setting accuracy of the bias potential output from the bias potential provider may be lower than the drive signal of the head driver. In such a case, a D/A converter to be incorporated in the bias potential provider can be embodied by a more compact and less-expensive D/A converter. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0051]     The above objects and advantages of the present invention will become more apparent by describing in detail preferred exemplary embodiments thereof with reference to the accompanying drawings, wherein:  
         [0052]      FIG. 1  is a block diagram showing a head driving device according to a first embodiment of the invention;  
         [0053]      FIG. 2  is a timing chart showing operation of the head driving device to be performed at commencement of printing operation;  
         [0054]      FIG. 3  is a timing chart showing operation of the head driving device to be performed during the course of printing operation;  
         [0055]      FIG. 4  is a timing chart showing operation of the head driving device to be performed at the end of the printing operation;  
         [0056]      FIG. 5  is a fragmentary circuit diagram showing an exemplary configuration of an analog switch in the head driving device;  
         [0057]      FIG. 6  is a block diagram showing a head driving device according to a second embodiment of the invention;  
         [0058]      FIG. 7  is a block diagram showing a head driving device according to a third embodiment of the invention;  
         [0059]      FIG. 8  is a timing chart showing a relationship between a drive signal of a head driver and a bias voltage in the head driving device shown in  FIG. 7 ;  
         [0060]      FIG. 9  is a flowchart showing operation of the head driving device shown in  FIG. 7  to be performed when the device is activated;  
         [0061]      FIG. 10A  is a timing chart showing a drive signal of the head driver of the head driving device shown in  FIG. 7 ;  
         [0062]      FIG. 10B  is a timing chart showing a bias voltage of the bias potential supplier of the head driving device shown in  FIG. 7 ;  
         [0063]      FIG. 11  is a flowchart showing operation of the head driving device shown in  FIG. 7  to be performed at commencement of printing operation; and  
         [0064]      FIG. 12  is a flowchart showing operation of the head driving device shown in  FIG. 7  to be performed when the device is deactivated.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0065]     Preferred embodiments of the invention will be described by reference to the accompanying drawings. The embodiments to be described hereinbelow are preferred specific embodiments of the invention, and hence technically-preferable limitations are imposed on the embodiments. However, the scope of the invention is not limited to the embodiments unless the following descriptions include descriptions which particularly specify the invention.  
         [0066]     As shown in  FIG. 1 , a head driving device  10  according to a first embodiment of the invention comprises: piezoelectric elements  11  provided so as to correspond to a plurality of nozzles of an ink jet printer; a head driver  12  for supplying a drive signal to electrodes  11   a  of the respective piezoelectric elements  11 ; a current amplifier  13  and a switcher  14 , both being interposed between the head driver  12  and the respective piezoelectric elements  11 ; and a bias potential provider  20  for applying an intermediate potential to ground-side electrodes  11   b  of the piezoelectric elements  11 .  
         [0067]     A row of nozzles are actually provided on a per-color basis in a print head of the ink jet printer  10 , and the piezoelectric elements  11  are provided for each of the rows of nozzles.  
         [0068]     The piezoelectric elements  11  are embodied by, e.g., elements exhibiting the piezoelectric effect and formed so as to become displaced by a voltage applied across the electrodes  11   a  and  11   b.    
         [0069]     The piezoelectric elements  11  remain charged in the vicinity of an intermediate potential Vc at all times. The piezoelectric elements  11  are arranged so as to eject ink droplets from nozzles by applying pressure to the ink stored in corresponding nozzles when performing discharging operation in accordance with a drive signal COM output from the head driver  12 .  
         [0070]     The head driver  12  is embodied as a driver IC and generates a drive signal COM to be sent to the print head which is placed in, e.g., a main unit of the printer.  
         [0071]     The current amplifier  13  is formed from two transistors  15 ,  16 . Of the transistors, a collector of the first transistor  15  is connected to a constant voltage power source (e.g., +42V DC power supply), and a base of the same is connected to one output terminal of the head driver  12 . Further, an emitter of the first transistor  15  is connected to an input terminal of the switcher  14 . As a result, in accordance with a signal output from the head driver  12 , a constant voltage Vcc is supplied to the piezoelectric elements  11  via the switcher  14 .  
         [0072]     An emitter of a second transistor  16  is connected to an input terminal of the switcher  14 . A base of the second transistor  16  is connected to a second output terminal of the head driver  12 . Further, a collector of the second transistor  16  is connected to ground. As a result, in accordance with a signal output from the head driver  12 , the piezoelectric elements  11  are caused to discharge by way of the switcher  14 .  
         [0073]     Upon receipt of a control signal, the switcher  14  Is turned on at a timing at which a corresponding piezoelectric element  11  is to be activated, thereby outputting the drive signal COM to that piezoelectric element  11 .  
         [0074]     The switcher  14  is actually formed as a so-called transmission gate for activating or deactivating the respective piezoelectric elements  11 .  
         [0075]     The bias potential provider  20  is constituted of a capacitor  21  serving as a potential applier, and a charger  22 .  
         [0076]     The capacitor  21  is an electrolytic capacitor. One end of the capacitor  21  is connected to the ground-side common electrodes  11   b  of the piezoelectric elements  11 , and the other end of the capacitor  21  is connected to ground such that a charging voltage of the capacitor; i.e., an intermediate potential Vc, is applied to the grounded elements  11   b  of the respective piezoelectric elements  11 .  
         [0077]     The capacitance of the capacitor  21  is selected so as to assume sufficient capacitance with respect to a total amount of electrostatic capacitance of all the piezoelectric elements  11  (a total of several microfarads; e.g., approximately 1.4 μF); that is, hundreds of microfarads to thousands of microfarads, so that the stable intermediate potential can be supplied to the respective piezoelectric elements  11 . Here, a device other than a capacitor may be employed as the potential applier.  
         [0078]     The charger  22  comprises a transistor  23  serving as a switching element; a resistor  24 ; a capacitor  25 ; and an analog switch  26 .  
         [0079]     An emitter of the transistor  23  is connected to one end of the capacitor  21 , and a collector of the same is connected to a constant voltage power supply Vcc.  
         [0080]     In lieu of the transistor  23 , any of various types of switching elements; for example, an FET, a thyristor, and a TRIAC, may also be employed.  
         [0081]     The resistor  24  is connected to a point located between the emitter of the transistor  23  and the ground. The capacitor  25  is connected to a point located between the base of the transistor  23  and the ground.  
         [0082]     Further, the analog switch  26  is connected to a point located between the base of the transistor  23 , and the emitter of the first transistor  15  and the emitter of the second transistor  16 , where the transistors  15 ,  16  belong to the current amplifier  13 .  
         [0083]     Upon receipt of an activation/deactivation control signal output from the control section of the printer main unit, the analog switch  26  is activated by, for example, a high-level control signal or deactivated by, for example, a low-level control signal.  
         [0084]     The control signal is set so as to be brought to a high level during a non-driving period of the drive signal COM output from the head driver  12  via the current amplifier  13 ; that is, a period of an intermediate potential, and so as to be brought to a low level during a driving period of the drive signal.  
         [0085]     The control signal is set so as to become continuously high at the commencement or end of printing operation.  
         [0086]     The head driving device  10  of the embodiment is constructed in the manner set forth and operates in the following manner in accordance with a head driving method of the invention.  
         [0087]     First, the operation of the head driving device  10  to be performed at start of printing operation of the ink jet printer (e.g., activation of the ink jet printer) will be described.  
         [0088]     At the time of commencement of printing operation, the drive signal COM output from the head driver  12  via the current amplifier  13  increases gradually.  
         [0089]     As a result, in accordance with the drive signal COM, an electric current flows from the first transistor  15  of the current amplifier  13  to the electrodes  11   a  of the piezoelectric elements  11  via the switcher  14 . As indicated by solid line “a” shown in  FIG. 2 , the electrodes  11   a  of the piezoelectric elements  11  gradually increase in potential up to the intermediate potential Vc; e.g., after a period of 20 μsec.  
         [0090]     At this time, as a result of activation of the analog switch  26 , the drive signal COM is applied to the base of the transistor  23  of the charger  22 , thereby activating the transistor  23 .  
         [0091]     As a result, a constant voltage output from the constant voltage power supply Vcc is applied to the capacitor  21 , thereby gradually charging the capacitor  21 . Accordingly, a charging voltage of the capacitor  21  gradually increases up to the intermediate potential Vc. As indicated by dashed lines “b” shown in  FIG. 2 , the ground-side electrodes  11   b  of the piezoelectric elements  11  also gradually increase in potential, thus reaching the intermediate potential Vc.  
         [0092]     In this way, the ground-side electrodes  11   b  of the piezoelectric elements  11  reach the intermediate potential in the same manner as do the electrodes  11   a  to be activated by the drive signal COM. Hence, a potential difference between the electrodes  11   a ,  11   b  of the piezoelectric elements is suppressed to a low level. Accordingly, since the potential difference is lower than the intermediate potential Vc of the drive signal COM, there is prevented ejection of ink droplets, which would otherwise be caused by faulty operation of the piezoelectric elements  11 .  
         [0093]     Operation of the head driving device  10  to be performed during printing operation of the ink jet printer will now be described. As shown in  FIG. 3 , when the drive signal COM is higher than the intermediate potential, the electrodes  11   a  of the piezoelectric elements  11  are charged by way of the first transistor  15  of the current amplifier  13  in accordance with fluctuations in the drive signal COM. When the drive signal COM is lower than the intermediate potential, the electrodes  11   a  of the piezoelectric elements  11  discharge an electric current via the second transistor  16  of the current amplifier  13 . As a result, the piezoelectric elements  11  operate in accordance with the drive signal COM, thereby ejecting ink droplets.  
         [0094]     At that time, as shown in  FIG. 3 , the analog switch  26  is activated only during the non-driving period of the drive signal COM (i.e., when the potential of the drive signal becomes the intermediate potential). Hence, the charger  22  always charges the capacitor  21  of the bias potential provider  20  with the intermediate potential.  
         [0095]     As a result, the intermediate potential Vc is applied to the common electrodes  11   b  of the piezoelectric elements  11  from the capacitor  21 . Hence, the electrodes  11   b  are always held at the intermediate potential Vc as indicated in dashed lines “b” shown in  FIG. 3 .  
         [0096]     Operation of the head driving device  10  to be performed at the end of the printing operation of the ink jet printer (e.g., when the ink jet printer is deactivated) will now be described.  
         [0097]     At the time of completion of printing operation, the drive signal COM to be output from the head driver  12  to the current amplifier  13  is discharged from the electrodes  11   a  of the piezoelectric elements  11  via the second transistor  16  of the current amplifier  13 , whereby the electrodes  11   a  fall to zero potential.  
         [0098]     At this time, the analog switch  26  is turned on, whereby the drive signal COM is applied to the base of the transistor  23  of the charger  21 . However, since the drive signal COM is in the midst of a gradual fall, the transistor  23  remains deactivated.  
         [0099]     The capacitor  21  of the bias potential provider  20  is grounded via the resistor  24 . Therefore, the capacitor  21  is gradually discharged. Since the charging voltage of the capacitor  21  falls to zero, the electrodes  11   b  of the piezoelectric elements  11  also gradually fall in potential, as indicated by dashed lines “b” shown in  FIG. 4 , to thereby reach zero.  
         [0100]     The ground-side electrodes  11   b  of the piezoelectric elements  11  gradually reach zero potential as in the case of the electrodes  11   a  to be activated by the drive signal COM. Therefore, a potential difference between the electrodes  11   a ,  11   b  of the piezoelectric elements is suppressed to a low level. Accordingly, the potential difference is lower than the intermediate potential Vc of the drive signal COM, and hence there is prevented ejection of ink droplets, which would otherwise be caused by faulty operation of the piezoelectric elements  11 .  
         [0101]     In this way, the power to be dissipated by the piezoelectric elements  11  is diminished, and a voltage drop stemming from spontaneous discharge of the piezoelectric elements is small, which in turn reduces a power loss.  
         [0102]     A potential difference between the piezoelectric elements  11  to be driven and the piezoelectric elements  11  not to be driven becomes small. Hence, even when these piezoelectric elements  11  are located adjacent to each other, electric discharge arising between the piezoelectric elements  11  is diminished. Moreover, even when the withstand voltage of each of the piezoelectric elements  11  becomes lower as a result of an increase in arrangement density, providing insulation between the piezoelectric elements  11  is unnecessary. Hence, an increase in arrangement density of a head can be achieved easily.  
         [0103]     Since the capacitor  21  is charged by utilization of a head drive voltage, a specific power supply circuit to be used for producing the intermediate potential Vc is not required.  
         [0104]      FIG. 5  shows an exemplary configuration of a switcher which can be used in place of the analog switch  26 .  
         [0105]     As shown in  FIG. 5 , a switcher  30  comprises, in lieu of the analog switch  26 , a transistor  31  connected to a point located between the base of the transistor  23 , the emitter of the first transistor  15 , and the emitter of the second transistor  16 , both transistors  15 ,  16  belonging to the current amplifier  13 ; and a transistor  32  connected to a point located between the base of the transistor  31  and the ground by way of a resistor  33 .  
         [0106]     A resistor  34  is connected to the base and emitter of the transistor  31 .  
         [0107]     An activation/deactivation control signal output from the control section of the printer main unit is input to the base of the transistor  32 .  
         [0108]     By the switcher  30  of such a configuration, as a result of a high-level control signal being input to the base of the transistor  32 , the drive signal COM flows to the ground via the resistors  33 ,  34 , thereby applying a voltage to the base of the transistor  31 . Thus, the transistor  31  is activated.  
         [0109]     As a result of a low-level control signal being input to the base of the transistor  32 , the potential of the base of the transistor  31  and the potential of the emitter of the transistor  31  are held at the same potential, and consequently the transistor  31  is deactivated.  
         [0110]     Activation and deactivation of the switcher  30  are controlled by the control signal in the same manner as employed for the analog switch  26 .  
         [0111]     As shown in  FIG. 6 , a head driving device  40  according to a second embodiment of the invention is substantially identical in configuration with the head driving device  10  shown in  FIG. 1 . Those constituent elements which are the same as those of the head driving device  10  are assigned the same reference numerals, and their explanations are omitted.  
         [0112]     As in the case of the head driving device  10  shown in  FIG. 1 , the head driver  12 , the current amplifier  13 , the switcher  14 , and the bias potential provider  20  are mounted on a print head  41  (or a carriage supporting a print head  17 ).  
         [0113]     The analog switch  26  of the bias potential provider  20  is constituted by utilization of an unused switching section of the switcher  14  mounted on the print head  41 .  
         [0114]     The head driving device  40  of such a configuration operates in the same manner as does the head driving device  10  shown in  FIG. 1 . Since the analog switch  26  utilizes an unused switch section of the switcher  14 , a smaller number of parts are required, whereby the cost of parts and an assembly cost can be reduced.  
         [0115]     In the above embodiments, the charger  22  is constituted of the transistor  23 , the resistor  24 , the capacitor  25 , and the analog switch  26 . However, the charger is not limited to such a circuit. A charger of another arbitrary configuration can also be used, so long as the circuit can supply a constant voltage from the constant voltage power supply Vcc to the capacitor  21 .  
         [0116]     As shown in  FIG. 7 , a head driving device  100  according to a third embodiment of the invention comprises: piezoelectric elements  11  provided so as to correspond to a plurality of nozzles of an ink jet printer; a head driver  12  for supplying a drive signal to electrodes  11   a  of the respective piezoelectric elements  11 ; a current amplifier  13  and a switcher  14 , both being interposed between the head driver  12  and the respective piezoelectric elements  11 ; a bias potential provider  20  for applying a predetermined bias voltage to ground-side electrodes  11   b  of the piezoelectric elements  11 ; a control resistor  121 ; and a capacitor  122 . Those constituent elements which are the same as those of the head driving devices according to the above embodiments are assigned the same reference numerals, and their explanations are omitted.  
         [0117]     The head driver  12  is embodied as a driver IC  130  and generates a drive signal COM to be sent to the print head placed in, e.g., a main unit of the printer.  
         [0118]     In this case, the head driver  12  is constituted of a latch  12   a ; a D/A converter  12   b ; and an amplifier  12   c.    
         [0119]     In this embodiment, the latch  12   a  is arranged so as to receive 10-bit data signals DATA 0  to DATA 9  output from the control section of the printer main unit, and a clock signal is input to a clock terminal CLK 1  of the latch  12   a.    
         [0120]     In accordance with the data signals DATA 0  to DATA 9  input to the D/A converter  12   b  by way of the latch  12   a , the D/A converter  12   b  outputs an analog signal corresponding to a drive voltage through D/A conversion.  
         [0121]     Further, the amplifier  12   c  amplifies the analog signal output from the D/A converter  12   b , to thereby produce a predetermined drive voltage waveform.  
         [0122]     The bias potential provider  20  is formed from a latch  123 , a D/A converter  124 , and an amplifier  125  in the same manner as is the head driver  12 .  
         [0123]     In the case of the illustrated embodiment, the latch  123  receives the 10-bit data signals DATA 0  to DATA 9  output from the control section of the printer main unit of the ink jet printer, and a clock signal is input to a clock terminal CLK 2  of the latch  123 .  
         [0124]     In accordance with the data signals DATA 0  to DATA 9  input by way of the latch  123 , through D/A conversion the D/A converter  124  outputs an analog voltage corresponding to the bias voltage.  
         [0125]     Further, the amplifier  125  amplifies an analog voltage output from the D/A converter  124 , thus producing a predetermined bias voltage.  
         [0126]     The bias potential provider  20  constituted of the latch  123 , the D/A converter  124 , and the amplifier  125  is housed in the driver IC  130  constituting the head driver  12  and embodied as a single IC package.  
         [0127]     In this way, the bias potential provider  20  outputs, to the ground-side electrodes  11   b  of the piezoelectric elements  11 , a predetermined bias voltage Vb, preferably a voltage substantially equal to the intermediate potential Vc of the drive signal COM output from the head driver  12 , as shown in  FIG. 8 .  
         [0128]     The control resistor  121  is a so-called coupling resistor and charges the capacitor  122  with the bias voltage Vb output from the bias potential provider  20 . At the time of discharging operation of the capacitor  122 , the control resistor  121  limits the current discharged from the capacitor  122 .  
         [0129]     The control resistor  121  is set to hundreds of ohms (e.g., 200Ω) so as to enable smooth charging of the capacitor  122  and to effectively limit a discharge current.  
         [0130]     The capacitor  122  is an electrolytic capacitor. One end of the capacitor  122  is connected to the ground-side common electrodes  11   b  of the piezoelectric elements  11 , and the other end of the capacitor  122  is grounded such that a charging voltage of the capacitor; i.e., the bias voltage Vb, is applied to the common electrodes  11   b  of the respective piezoelectric elements  11 .  
         [0131]     The capacitance of the capacitor  122  is selected so as to assume sufficient capacitance with respect to a total amount of electrostatic capacitance of all the piezoelectric elements  11  (a total of several microfarads; e.g., approximately 1.4 μF); that is, thousands of microfarads (e.g., approximately 3300 μF) so that the stable bias voltage Vb can be supplied to the respective piezoelectric elements  11 .  
         [0132]     The head driving device  100  of the embodiment is constructed in the manner set forth and operates in the following manner.  
         [0133]     First, operation to be performed at the time of activation of the ink jet printer will be described in accordance with a flowchart shown in  FIG. 9 .  
         [0134]     When the ink jet printer is activated, the control section of the printer main unit detects a head temperature (step A 1 ), and calculatively determines an intermediate voltage Vc 1  corresponding to the thus-detected temperature (step A 2 ). Incidentally, the temperature detected in the step A 1  may be a temperature in the vicinity of the print head, an environmental temperature of the printer, or the like.  
         [0135]     Subsequently, the control section of the printer main unit activates all nozzles of the printer head (step A 3 ). In step A 4 , the control section gradually increases digital values represented by the data signals DATA 0  to DATA 9  while delivering a clock signal to the clock terminal CLK 1 , thus controlling the D/A converter of the head driver  12 .  
         [0136]     As a result, by way of the switcher  14  an electric current flows from the first transistor  15  of the current amplifier  13  in response to the drive signal COM, thereby charging the electrodes  11   a  of the piezoelectric elements  11 . As indicated by reference symbol A shown in  FIG. 10A , the electrodes  11   a  of the piezoelectric elements  11  increase up to the intermediate potential Vc 1 .  
         [0137]     Subsequently, the control section of the printer main unit outputs a digital value of the intermediate potential Vc 1  in the form of the data signals DATA 0  to DATA 9  (step A 5 ). In step A 6 , the control section outputs one clock pulse to the CLK 2  terminal of the latch  123  of the bias potential provider  20 , thereby controlling the D/A converter  124  of the bias potential provider  20 .  
         [0138]     As a result, the bias potential provider  20  applies a bias voltage Vb (=Vc 1 ) to the capacitor  122  by way of the control resistor  121 , thus charging the capacitor  122 . The charging voltage of the capacitor  20  gradually increases up to the intermediate potential Vc 1  in accordance with a time constant defined by the control resistor  121  and the capacitor  122 . As indicated by reference symbol B shown in  FIG. 10B , the potential of the ground-side electrodes  11   b  of the piezoelectric elements  11  gradually increases and finally reaches the intermediate potential Vc 1 . Accordingly, a potential difference between the electrodes  11   a ,  11   b  of the piezoelectric elements becomes substantially zero. At this point, the operation of the printer driver to be performed at the activation is completed.  
         [0139]     The bias voltage Vb stored in the capacitor  122  is applied to the ground-side electrodes  11   b  of the piezoelectric elements  11 . Hence, the amplifier  125  of the bias potential provider  20  does not need to be a high-speed operable type; an amplifier which outputs a small electric current will be sufficient.  
         [0140]     Next, the operation of the head driving device to be performed at the commencement of printing operation will now be described by reference to a flowchart shown in  FIG. 11 . In accordance with the flowchart shown in  FIG. 11 , when commencement of printing operation of the ink jet printer is instructed, the control section of the printer main unit detects a temperature (step B 1 ), and calculatively determines an intermediate voltage Vc 2  corresponding to the thus-detected temperature (step B 2 ). Incidentally, the temperature detected in the step B 1  may be a temperature in the vicinity of the print head, an environmental temperature of the printer, or the like.  
         [0141]     Subsequently, the control section of the printer main unit activates all the nozzles of the printer head (step B 3 ). In step B 4 , the digital value represented by the data signals DATA 0  to DATA 9  is caused to change gradually. As a result of the clock signal being input to the clock terminal CLK 1 , the D/A converter  12   b  of the head driver  12  is controlled.  
         [0142]     As a result, when Vc 1 &lt;Vc 2 , an electric current flows into the electrodes  11  a of the piezoelectric elements  11  from the first transistor  15  of the current amplifier  13  by way of the switcher  14  in accordance with the drive signal COM, thereby charging the electrodes  11   a . As indicated by reference symbol C shown in  FIG. 10A , the voltage of the electrodes  11   a  reaches the intermediate potential Vc 2 . When Vc 1 &gt;Vc 2 , an electric current is discharged from the electrodes  11  a of the piezoelectric elements  11  by way of the second transistor  16  of the current amplifier  13 , whereby the piezoelectric elements  11  are operated in accordance with the drive signal COM, thus ejecting ink droplets.  
         [0143]     Subsequently, the control section of the printer main unit outputs a digital value of the intermediate potential Vc 2  in the form of the data signals DATA 0  to DATA 9  (step B 5 ). In step B 6 , the control section outputs one clock pulse to a CLK 2  terminal of the latch  123  of the bias potential provider  20 , thus controlling the D/A converter  124  of the bias potential provider  20 .  
         [0144]     As a result, the bias potential provider  20  applies the bias voltage Vb (=Vc 2 ) to the capacitor  122  by way of the control resistor  121 , thereby charging the capacitor  122  Eventually, a charging voltage of the capacitor  20  gradually changes up to the intermediate voltage Vc 2  on the basis of the time constant defined by the control resistor  121  and the capacitor  122 . As indicated by reference symbol D shown in  FIG. 10B , the potential of the ground-side electrodes  11   b  of the piezoelectric elements  11  also changes gradually, to thereby reach the intermediate potential Vc 2 . Accordingly, a potential difference between the electrodes  11   a ,  11   b  of the piezoelectric elements becomes substantially zero. At this point, the operation of the head driving device to be performed at the commencement of the printing operation is completed.  
         [0145]     When printing operation is performed in this state, the electrodes  11   a  of the piezoelectric elements  11  are charged by way of the first transistor  15  of the current amplifier  13  in accordance with variations in the drive signal COM during a period in which the voltage of the drive signal COM is increasing. During a period in which the voltage of the drive signal COM Is decreasing, the electrodes  11   a  of the piezoelectric elements  11  discharge an electric current by way of the second transistor  16  of the current amplifier  13 . As a result, the piezoelectric elements  11  operate in accordance with the drive signal COM, thereby ejecting ink droplets.  
         [0146]     Next, the operation of the head driving device to be performed at the deactivation will be described in accordance with a flowchart shown in  FIG. 12 . When the deactivation of the ink jet printer is instructed, the control section of the printer main unit activates all the nozzles of the printer head (step C 1 ). In step C 2 , the control section sets the data signals DATA 0  to DATA 9  to zero. In step C 3 , one clock pulse is provided to the clock terminal CLK 2  of the latch  123  of the bias potential provider  20 .  
         [0147]     As a result, the D/A converter  124  of the bias potential provider  20  outputs an analog signal corresponding to a bias voltage Vb=0. Hence, the amplifier  125  outputs a zero bias voltage.  
         [0148]     Eventually, the capacitor  122  is discharged. The electric current discharged from the capacitor  122  is gradually discharged from the bias potential provider  20  to the ground while passing through the control resistor  121 . In association with this discharging operation, the potential of the ground-side electrodes  11   b  of the piezoelectric elements  11  also falls to zero as indicated by symbol E shown in  FIG. 10B .  
         [0149]     Subsequently, after elapse of a preset given period of time required for causing the capacitor  122  to discharge (step C 4 ), the control section of the printer main unit gradually decreases the digital value represented by the data signals DATA 0  to DATA 9  (step C 5 ). The control section controls the D/A converter of the head driver  12  by inputting a clock signal to the clock terminal CLK 1 .  
         [0150]     As a result, an electric current flows from the electrodes  11   a  of the piezoelectric elements  11  to the ground by way of the switcher  14  and the second transistor  16  of the current amplifier  13 . As indicated by reference symbol F shown in  FIG. 10A , the potential of the electrodes  11   a  of the piezoelectric elements  11  falls to zero.  
         [0151]     As a result of the potential of the electrodes  11   a  of the piezoelectric elements  11  and that of the electrodes  11   b  of the same having dropped to zero, the operation of the head driving device to be performed at the deactivation is completed, and subsequently power is turned off.  
         [0152]     In this way, the potential of the ground-side electrodes  11   b  of the respective piezoelectric elements  11  is held at the bias voltage Vb; preferably, the intermediate potential Vc, by the charging voltage of the capacitor  122  supplied from the bias potential provider  20 . Hence, the potential difference between the electrodes  11   a ,  11   b  of the piezoelectric elements  11  is held at substantially zero. When piezoelectric elements to be driven and piezoelectric elements not to be driven are located adjacent to each other, a potential difference across the electrodes  11   a  of the piezoelectric elements  11  is also held substantially at zero.  
         [0153]     A voltage drop stemming from self-discharge of the piezoelectric elements  11  is small, thereby diminishing a power loss.  
         [0154]     A potential difference between the piezoelectric elements  11  to be driven and the piezoelectric elements  11  not to be driven becomes low.  
         [0155]     Hence, even when these piezoelectric elements  11  are located adjacent to each other, electric discharge arising between the piezoelectric elements  11  is diminished. Moreover, even when the withstand voltage of each of the piezoelectric elements  11  becomes lower as a result of an increase in arrangement density, provision of insulation between the piezoelectric elements  11  is not required. Hence, an increase in arrangement density of a head can be easily achieved.  
         [0156]     The bias potential provider  20  is constituted integrally with the head driver  12  as a single driver IC  130 . Hence, only a small packing space is required. Moreover, both data signals to be input to the bias potential provider  20  and those to be input to the head driver  12  are 10-bit common data signals. Hence, smaller wiring and connection space is sufficient.  
         [0157]     A bias voltage of the bias potential provider  20  is applied to the capacitor  122  by way of the control resistor  121 . The amplifier  125  of the bias potential provider  20  does not need to be a high-speed operable type; a low-cost, small-capacity amplifier can be employed.  
         [0158]     The electric current discharged from the capacitor  122  is limited by the control resistor  121 , thereby preventing flow of a large electric current into the bias potential provider  20 . Hence, the amount of heat dissipated by the amplifier  125  of the bias potential provider  20  can be greatly reduced.  
         [0159]     In the embodiment, the bias potential provider  20  outputs a bias voltage Vb equal to the intermediate voltage Vc of the drive signal COM output from the head driver  12 . However, the bias potential provider  20  may output a bias voltage Vb offset from the intermediate voltage Vc.  
         [0160]     In this case, a potential between the electrodes  11   a ,  11   b  of the piezoelectric elements  11  does not become substantially zero. However, when compared with a case where the bias voltage is not employed, the potential difference becomes smaller, thereby reducing power to be consumed by the piezoelectric elements. Moreover, a voltage drop stemming from spontaneous discharge of the piezoelectric elements becomes smaller, thereby reducing a power loss. Occurrence of electric discharge resulting from a potential difference between the piezoelectric elements to be driven and the piezoelectric elements not to be driven is also diminished. Even when the piezoelectric elements are made compact and their withstand voltages become lower, the piezoelectric elements can cope with the drive signal. Hence, the arrangement density of the piezoelectric elements can be made increased further without involvement of an operation for providing insulation between electrodes of the piezoelectric elements.  
         [0161]     In the embodiments, the 10-bit data signals DATA 0  to DATA 9  are input to the bias potential provider  20 , as in the case of the head driver  12 . However, data signals of smaller bits may also be employed.  
         [0162]     In this case, the bias voltage may be in the vicinity of an intermediate voltage of the drive signal. Further, the bias voltage may also be less accurate than the drive signal. Hence, for example, an 8-bit data signal may be employed, so long as the maximum value and resolution of the bias voltage are halved. Accordingly, use of an 8-bit latch  123  and an 8-bit D/A converter  124  leads to cost reduction.  
         [0163]     Although all the nozzles are turned on in step A 3  shown in  FIG. 9 , in step B 3  shown in  FIG. 11 , and in step C 1  shown in  FIG. 12 , all the nozzles may be deactivated. In this case, substantially no current flows through the two transistors  15 ,  16  of the current amplifier  13 , thus yielding the same result. Moreover, activation or deactivation of the nozzles does not need to be determined. However, in this case, there arises a problem of failure to determine an electric current to flow in a charging/discharging process.  
         [0164]     In the above embodiments, the piezoelectric elements  11  are embodied by elements exhibiting the piezoelectric effect. However, other elements; e.g., electrostrictive elements or magnetostrictive elements, may be employed.  
         [0165]     The invention can be also applied to display manufacturing apparatuses, electrode forming apparatuses, biochip manufacturing apparatuses, or various types of liquid jetting apparatuses, as well as ink jet printers. Furthermore, the invention can be also applied to a jetting apparatus in which any kinds of gas is selected as a jetted object.