Abstract:
A head drive device of an inkjet printer having a nozzles and corresponding actuators that jet liquid drops. A drive section applies a drive signal to the actuators. The head drive device includes a drive waveform generating section that generates a drive waveform signal which is used as a reference of a signal to control drive of the actuators. A modulating section modulates a pulse of a drive waveform signal generated by the drive waveform generating section. A digital power amplifier amplifies power of a modulated signal subjected to the pulse modulation. A low pass filter smoothes a power-amplified modulated signal subjected to the power amplification by the digital power amplifier and supplies the signal as a drive signal to the actuators. A frequency characteristics adjusting section adjusts frequency characteristics of the low pass filter as a function of the number of the actuators.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to an inkjet printer in which a plurality of nozzles jet minute ink drops of liquid ink of a plurality of colors and particles of the ink (ink dots) are formed on a print medium to draw pre-determined characters and images. 
         [0003]    2. Related Art 
         [0004]    An inkjet printer as in the above generally accomplishes low-cost and high-quality color printed material easily. As such, it is widely used not only in offices but also by general users along with popularization of a personal computer and a digital camera. 
         [0005]    Generally, in such an inkjet printer, a moving part called a carriage, for example, integrally comprising ink cartridges and print heads moves back and forth on a print medium in a direction crossing a direction to convey the medium, and nozzles of the print head jet (eject) liquid ink drops to form minute ink dots on the print medium. In this manner, pre-determined characters or images are drawn on the print medium to create desired printed material. The carriage comprises ink cartridges for four colors including black (and yellow, magenta, cyan) and a print head for each of the colors, so that not only monochrome print but also full color print in combination of the respective colors can be easily performed (further, print in six colors including the colors, light cyan and light magenta, seven colors, and eight colors are practically implemented). 
         [0006]    In the above type of inkjet printer for executing print by moving the inkjet heads on the carriage back and forth in a direction crossing a direction to convey a print medium in the above manner, the inkjet heads must be moved back and forth about ten times to more than tens of times to neatly print a whole page. Therefore, it has a drawback in that it takes a longer time for printing than a print apparatus in another scheme, for example, a laser printer or a copying machine using electrographic technique. 
         [0007]    On the other hand, in an inkjet printer comprising inkjet heads (do not need to be integrated) of the same length as the width of a print medium but not comprising a carriage, the inkjet heads do not need to be moved in a width direction of the print medium so that one-pass printing is possible, enabling quick printing similar to a laser printer. An inkjet printer in the former scheme is generally called a “multi-pass (serial) inkjet printer”, while an inkjet printer in the latter scheme is generally called a “line head inkjet printer”. 
         [0008]    The above types of inkjet printers are required to provide further higher gradation. Gradation is the density of each color included in a pixel represented by an ink dot: the size of an ink dot depending on the density of a color of each pixel is called gradient, while the number of gradients represented by an ink dot is called the number of gradations. High gradation means that the number of gradations is large. To change gradient, it is necessary to change a drive pulse to an actuator provided to an inkjet head. For example, if an actuator is a piezoelectric element, when a voltage value applied to the piezoelectric element is large, the magnitude of displacement (distortion) of the piezoelectric element (precisely, a vibrating plate) is also large. This is used to change the gradient of an ink dot. 
         [0009]    According to JP-A-10-81013, a plurality of drive pulses having different voltage peak values are combined and coupled to generate a drive signal. The signal is output commonly to piezoelectric elements of nozzles for the same color provided to an inkjet head. According to the drive signal, a drive pulse for the gradient of an ink dot to be formed is selected for each nozzle. The selected drive pulse is supplied to a piezoelectric element of an appropriate nozzle to jet an ink drop. In this manner, a requested gradient of an ink dot is achieved. 
         [0010]    A method for generating a drive signal (or drive pulse) is illustrated in FIG. 2 of JP-A-2004-306434. That is, data is read out from a memory for storing drive signal data, a D/A converter converts the data into analog data, and a drive signal is supplied to an inkjet head through a current amplifier. A circuit of the current amplifier comprises transistors in push-pull connection, as shown in  FIG. 3  of the document, in which a linear drive amplifies a drive signal. However, in a current amplifier with such configuration, a linear drive itself of a transistor is inefficient. Moreover, such an amplifier has a drawback of a large circuit size since the transistor itself should be large for a countermeasure against heat, and the transistor needs a cooling plate radiator. Particularly, the largeness of the cooling plate radiator is a major obstacle to the layout. 
         [0011]    To resolve the drawback, JP-A-2005-035062 discloses an inkjet printer for generating a drive signal by controlling the reference voltage of a DC/DC converter. According to the document, an efficient DC/DC converter is used to dispense with a radiating unit for cooling. Additionally, a PWM signal is used so that a D/A converter can be realized using a simple low-pass filter. These can realize a small circuit. 
         [0012]    However, a DC/DC converter is originally designed to generate a constant voltage. As such, the head drive apparatus of an inkjet printer using the DC/DC converter in JP-A-2005-035062 has a problem in that a waveform, for example, rapid rise and fall of a drive signal cannot be gained necessary for an inkjet head to jet ink drops well. Of course, the head drive apparatus of an inkjet printer in which a push-pull connection of transistors amplifies current of an actuator drive signal in JP-A-2004-306434 has a problem in that a cooling plate radiator is so large that it cannot be actually laid out particularly in a line head inkjet printer having a large number of nozzles, i.e., a large number of actuators. 
       SUMMARY 
       [0013]    An object of the present invention is to provide a head drive apparatus of an inkjet printer that enables rapid rise and fall of a drive signal to an actuator, does not require a cooling unit such as a cooling plate radiator, and makes drive signals actually applied to actuators uniform. 
       FIRST ASPECT 
       [0014]    To solve the above problems, a head drive apparatus of an inkjet printer according to a first aspect is characterized by including: a plurality of nozzles for jetting liquid drops that are provided for an inkjet head; actuators provided in correspondence to the nozzles; and a drive unit that applies a drive signal to the actuators, and further including: a drive waveform generator that generates a drive waveform signal which is used as a reference of a signal to control drive of the actuators; a modulator that pulse modulates a drive waveform signal generated by the drive waveform generator; a digital power amplifier for amplifying power of a modulated signal subjected to the pulse modulation by the modulator; a low pass filter for smoothing an amplified digital signal subjected to the power amplification by the digital power amplifier and supplying the signal as a drive signal to the actuators; and a frequency characteristics adjusting unit that adjusts frequency characteristics of the low pass filter as a function of the number of the actuators. 
         [0015]    In the head drive apparatus of an inkjet printer according to the first aspect, the drive waveform generator generates a drive waveform signal which is used as a reference of a signal to control drive of the actuators, the modulator pulse modulates the generated drive waveform signal, the digital power amplifier amplifies the power of the modulated signal subjected to the pulse modulation, and the low pass filter smoothes the amplified digital signal subjected to the power amplification and supplies the signal as a drive signal to the actuator. Thus, filter characteristics of the low pass filter are set to sufficiently smooth only a amplified digital signal component so that rapid rise and fall of a drive signal to the actuators are enabled and the digital power amplifier with efficient power amplification can efficiently amplify the power of a drive signal. This allows the device to dispense with a cooling unit such as a cooling plate radiator. 
         [0016]    The head drive apparatus is configured so that frequency characteristics of the low pass filter is adjusted depending on the number of the actuators, thereby the low-pass filter in the drive circuit removes only certain components or only the components within a predetermined range, which makes drive signals actually applied to actuators constant. 
       SECOND ASPECT 
       [0017]    A head drive apparatus of an inkjet printer according to a second aspect of the present invention is characterized by that, in the head drive apparatus of an inkjet printer according to the first aspect, the frequency characteristics adjusting unit comprises: a plurality of capacitances which can be connected in parallel relative to the amplified digital signal; and switches for individually connecting to the plurality of capacitances to the amplified digital signal. 
         [0018]    According to a head drive apparatus of an inkjet printer of the second aspect of the present invention, since the head drive apparatus is configured to have a plurality of capacitances which can be connected in parallel to the amplified digital signal; and a switch for individually connecting the plurality of capacitances to the amplified digital signal, with the smaller number of the actuators and the larger capacitance which is connected in parallel to the amplified digital signal, the low-pass filter in the drive circuit can remove only certain components or only the components within a predetermined range, which can make drive signals actually applied to actuators uniform. 
       THIRD ASPECT 
       [0019]    A head drive apparatus of an inkjet printer according to a third aspect of the present invention is characterized by that, in the head drive apparatus of an inkjet printer according to the second aspect, the frequency characteristics adjusting unit increases the capacitance connected in parallel to the amplified digital signal for the smaller number of the actuators. 
         [0020]    According to the head drive apparatus of an inkjet printer of the third aspect of the present invention, since the head drive apparatus is configured so that the capacitance connected in parallel to the amplified digital signal is increased for the smaller number of the actuators, thereby the low-pass filter in the drive circuit can remove only certain components or only the components within a predetermined range, which can make drive signals actually applied to actuators uniform. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0021]      FIGS. 1A and 1B  are the overall configuration diagrams showing a line head inkjet printer to which a head drive apparatus of the inkjet printer according to the present invention is applied: (a) is a top plain view, and (b) is a front view; 
           [0022]      FIG. 2  is a block diagram of a control unit of the inkjet printer of  FIGS. 1A and 1B ; 
           [0023]      FIG. 3  is a block diagram of a drive waveform generator of  FIG. 2 ; 
           [0024]      FIG. 4  is a diagram illustrating a waveform memory of  FIG. 3 ; 
           [0025]      FIG. 5  is a diagram illustrating drive waveform signal generation; 
           [0026]      FIG. 6  is a diagram illustrating the drive waveform signals or drive signals connected sequentially in time; 
           [0027]      FIG. 7  is a block diagram of a drive signal output circuit; 
           [0028]      FIG. 8  is a block diagram of a selector for connecting a drive signal to an actuator; 
           [0029]      FIG. 9  is a block diagram showing details of a modulator, a digital power amplifier and a low pass filter of the drive signal output circuit of  FIG. 7 ; 
           [0030]      FIG. 10  is a diagram illustrating an operation of the modulator of  FIG. 9 ; 
           [0031]      FIG. 11  is a diagram illustrating an operation of the digital power amplifier of  FIG. 9 ; 
           [0032]      FIG. 12A  is a diagram illustrating a change in a drive signal depending on the number of connected actuators; and  12 B is a diagram illustrating frequency characteristics of a drive circuit; 
           [0033]      FIGS. 13A ,  13 B,  13 C and  13 D are the diagrams illustrating of a low-pass filter configured with connected actuators; 
           [0034]      FIG. 14  is a flowchart showing a calculation processing for setting a switch drive signal; 
           [0035]      FIG. 15  is a diagram illustrating a total capacitance of a drive circuit by the calculation processing of  FIG. 14 ; 
           [0036]      FIG. 16  shows another embodiment of a head drive apparatus of an inkjet printer according to the present invention, and is a block diagram of a drive waveform generator and a modulator thereof; and 
           [0037]      FIG. 17  shows another embodiment of a head drive apparatus of an inkjet printer according to the present invention, and is a block diagram of a low pass filter thereof. 
       
    
    
     DESCRIPTION OF SYMBOLS 
       [0038]      1 : print medium;  2 : first inkjet head;  3 : second inkjet head;  4 : first conveyor unit;  5 : second conveyor unit;  6 : first conveyor belt;  7 : second conveyor belt;  8 R and  8 L: drive rollers;  9 R and  9 L: first driven rollers;  10 R and  10 L: second driven rollers;  11 R and  11 L: electric motors;  24 : modulator;  25 : digital power amplifier;  26 : low pass filter;  31 : comparator;  32 : triangular wave oscillator;  33 : half bridge driver stage;  34 : gate drive circuit;  41 : memory controller;  42 : memory unit;  43 : numerical value generator;  44 : comparing unit;  70 : drive waveform generator. 
       DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0039]    A first embodiment of an inkjet printer according to the present invention will be described with reference to the drawings. 
         [0040]      FIGS. 1A and 1B  are the overall configuration diagrams of an inkjet printer according to this embodiment:  FIG. 1A  is a top plain view of the printer; and  FIG. 1B  is a front view of the printer. In  FIGS. 1A and 1B , a print medium  1  is a line head inkjet printer that is conveyed in a direction from the right to the left indicated by the arrow of the drawing and printed in a printing area on the way of the conveyor. However, the inkjet head according to the present embodiment is not arranged only at one place, but two inkjet heads are arranged at two places. 
         [0041]    Reference numeral  2  in the drawing denotes a first inkjet head being provided on the upstream side of the direction in which the print medium  1  is conveyed, and reference numeral  3  denotes a second inkjet head being provided on the downstream side of the direction. A first conveyor unit  4  is provided below the first inkjet heads  2  that carries the print medium  1 , while a second conveyor unit  5  is provided below the second inkjet heads  3 . The first conveyor unit  4  includes four first conveyor belts  6  which are arranged with predetermined space therebetween in the direction crossing the direction in which the print medium  1  is conveyed (hereinafter, also referred to as a nozzle array direction), and the second conveyor unit  5  similarly includes four second conveyor belts  7  which are arranged with predetermined space therebetween in the direction (nozzle array direction) crossing the direction in which the print medium  1  is conveyed. 
         [0042]    The four first conveyor belts  6  and the similar four second conveyor belts  7  are arranged alternately so as to be adjacent to each other. This embodiment divides the conveyor belts into two of the first conveyor belts  6  and two of the second conveyor belts  7  on the left side in the nozzle array direction, and two of the first conveyor belts  6  and two of the second conveyor belts  7  on the right side in the nozzle array direction. That is, a right drive roller  8 R is provided through an overlapping part of the two first conveyor belts  6  and the two second conveyor belts  7  on the right side in the nozzle array direction. A left drive roller  8 L is provided through an overlapping part of the two first conveyor belts  6  and the two second conveyor belts  7  on the left side in the nozzle array direction. A first right driven roller  9 R and a first left driven roller  9 L are provided on the upstream side, while a second right driven roller  10 R and a second left driven roller  10 L are provided on the downstream side. The rollers are practically separated at the center part of  FIG. 1A , though they individually seem to be continuous rollers. The two first conveyor belts  6  on the right side in the nozzle array direction are wound around the right drive roller  8 R and the first right driven roller  9 R, and the two first conveyor belts  6  on the left side in the nozzle array direction are wound around the left drive roller  8 L and the first left driven roller  9 L. The two second conveyor belts  7  on the right side in the nozzle array direction are wound around the right drive roller  8 R and the second right driven roller  10 R, the two second conveyor belts  7  on the left side in the nozzle array direction are wound around the left drive roller  8 L and the second left driven roller  10 L. The right drive roller  8 R is connected to the right electric motor  11 R, while the left drive roller  8 L is connected to the left electric motor  11 L. Therefore, when the right electric motor  11 R rotates the right drive roller  8 R, the first conveyor unit  4  having the two first conveyor belts  6  on the right side in the nozzle array direction and the second conveyor unit  5  similarly having the two second conveyor belts  7  on the right side in the nozzle array direction synchronize with each other and move at the same speed. When the left electric motor  11 L rotates the left drive roller  8 L, the first conveyor unit  4  having the two first conveyor belts  6  on the left side in the nozzle array direction and the second conveyor unit  5  similarly having the two second conveyor belts  7  on the left side in the nozzle array direction synchronize with each other and move at the same speed. However, if the right electric motor  11 R and the left electric motor  11 L rotate at different speeds, conveyor speeds on left and right sides in the nozzle array direction can be different from each other. Specifically, if the right electric motor  11 R rotates faster than the left electric motor  11 L, the conveyor speed of the right side in the nozzle array direction can be higher than that of the left side. If the left electric motor  11 L rotates faster than the right electric motor  11 R, the conveyor speed of the left side in the nozzle array direction can be higher than that of the right side. 
         [0043]    The first inkjet heads  2  and the second inkjet heads  3  are arranged offset from each other in the direction in which the print medium  1  is conveyed for each of four colors of yellow (Y), magenta (M), cyan (C) and black (K). To the respective inkjet heads  2  and  3 , ink is supplied from ink tanks (not shown) for the respective colors via ink supply tubes. Each of the inkjet heads  2  and  3  has a plurality of nozzles formed therein in the direction crossing the direction in which the print medium  1  is conveyed (i.e., the nozzle array direction). The nozzles simultaneously jet a necessary amount of ink drops to a necessary position to form and output minute ink dots on the print medium  1 . This is performed for each color so that only one pass of the print medium  1  conveyed by the first conveyor unit  4  and the second conveyor unit  5  enables one-pass printing thereon. That is, the areas where the inkjet heads  2  and  3  are arranged correspond to printing areas. 
         [0044]    A method for jetting and outputting ink from each nozzle of an inkjet head includes an electrostatic scheme, a piezoelectric inkjet, and a film-boiling ink jet. In the electrostatic scheme, an application of a drive signal to an electrostatic gap which functions as an actuator causes a displacement of a vibrating plate in a cavity and a pressure change in the cavity, which the causes ink drops to be jetted and output from a nozzle. In the piezoelectric inkjet, an application of a drive signal to a piezoelectric element which functions as an actuator causes a displacement of a vibrating plate in a cavity and a pressure change in the cavity, which causes ink drops to be jetted and output from a nozzle. In the film-boiling ink jet, a micro heater in a cavity is instantaneously heated to a temperature of 300 degrees or more, so as to cause a film-boiling state of ink and generate bubbles in the ink, resulting in a pressure change which causes ink drops to be jetted and output from a nozzle. The present invention can be applied to any of the above ink output methods, but among them, is particularly preferable to a piezoelectric element since the amount of ink drop ejection can be adjusted by controlling a peak voltage or a voltage gradient of a drive signal. 
         [0045]    The ink drop jetting nozzles of the first inkjet heads  2  are formed only between the four first conveyor belts  6  of the first conveyor unit  4 , while the ink drop jetting nozzles of the second inkjet heads  3  are formed only between the four second conveyor belts  7  of the second conveyor unit  5 . This allows a cleaning unit which will be described below to clean the respective inkjet heads  2  and  3 , but in this configuration, one-pass full-page printing cannot be accomplished only by either of the inkjet heads. Accordingly, in order to cover the areas where either of the inkjet heads cannot print, the first inkjet heads  2  and the second inkjet heads  3  are arranged offset from each other in the direction in which the print medium  1  is conveyed. 
         [0046]    A first cleaning cap  12  for cleaning the first inkjet heads  2  is provided under the first inkjet heads  2 , while a second cleaning cap  13  for cleaning the second inkjet heads  3  is provided under the second inkjet heads  3 . Both of the cleaning caps  12  and  13  are formed to have a size which can pass between the four first conveyor belts  6  of the first conveyor unit  4  and between the four second conveyor belts  7  of the second conveyor unit  5 , respectively. The cleaning caps  12  and  13  individually include: a square cap body with a bottom that covers the nozzles formed in the bottom surfaces of the inkjet heads  2  and  3 , i.e., the nozzle side surface, and can be adhered to the nozzle side surface; an ink absorber provided on the bottom thereof; a tube pump connected to the bottom of the cap body; and an elevator for moving up and down the cap body. Thus, the elevator moves up the cap body to adhere the body to each nozzle side surface of the inkjet heads  2  and  3 . When the tube pump creates a negative pressure in the cap body as such, ink drops and bubbles are sucked up through the nozzles which are open in the nozzle side surface of the inkjet heads  2  and  3 , which cleans the inkjet heads  2  or  3 . When the cleaning is finished, the cleaning caps  12  and  13  are moved down. 
         [0047]    On the upstream side of the first driven rollers  9 R and  9 L, a pair of gate rollers  14  is provided for controlling timing to feed the print medium  1  supplied from a paper feeder  15  and for correcting the skew of the print medium  1 . The skew is torsion of the print medium  1  relative to the conveyor direction. A pickup roller  16  for supplying the print medium  1  is provided above the paper feeder  15 . Reference numeral  17  in the drawing denotes a gate roller motor for driving the gate rollers  14 . 
         [0048]    A belt charging unit  19  is provided below the drive rollers  8 R and  8 L. The belt charging unit  19  includes: a charging roller  20  contacting the first conveyor belts  6  and the second conveyor belts  7  across the drive rollers  8 R and  8 L; a spring  21  for pressing the charging roller  20  against the first conveyor belts  6  and the second conveyor belts  7 ; and a power source  18  for imparting electric charge to the charging roller  20 , and the electric charge is imparted from the charging roller  20  to the first conveyor belts  6  and the second conveyor belts  7  for charging. Generally, when such a type of belt which includes a medium or high resistor or insulator is charged by the belt charging unit  19 , the electric charge transferred to the surface thereof induces polarization to the print medium  1  which also includes a high resistor or insulator. The electrostatic force between electric charge generated by the induced polarization and electric charge of the belt surface allows the print medium  1  to be adsorbed to the belt. The belt charging unit  19  may be a corotron which sprays electric charge. 
         [0049]    Therefore, according to the inkjet printer, the belt charging unit  19  charges the surfaces of the first conveyor belts  6  and the second conveyor belts  7 , and in the state, the gate rollers  14  feeds the print medium  1  to be pressed against the first conveyor belt  6  by a paper pressing roller which is configured with a spur or a roller (not shown). Then, the print medium  1  is adsorbed to the surface of the first conveyor belts  6  by the operation of the induced polarization described above. In this state, a rotation of the drive rollers  8 R and  8 L by the electric motors  11 R and  11 L causes the generated rotary drive force to be transmitted to the first driven rollers  9 R and  9 L via the first conveyor belts  6 . 
         [0050]    With the print medium  1  adsorbed as described above, the first conveyor belts  6  are moved downstream in the conveyor direction to cause the print medium  1  to be moved to a position under the first inkjet heads  2 , so that ink drops are jetted through the nozzles formed in the first inkjet head  2  for printing. When the printing by the first inkjet heads  2  is finished, the print medium  1  is moved downstream in the conveyor direction to be transferred to the second conveyor belts  7  of the second conveyor unit  5 . As described above, since the surfaces of the second conveyor belts  7  are also charged by the belt charging unit  19 , the operation of the induced polarization described above causes the print medium  1  to be adsorbed to the surfaces of the second conveyor belts  7 . 
         [0051]    In this state, the second conveyor belts  7  are moved downstream in the conveyor direction to cause the print medium  1  to be moved to a position under the second inkjet head  3 , so that ink drops are jetted through the nozzles formed in the second inkjet head for printing. When the printing by the second inkjet head is finished, the print medium  1  is further moved downstream in the conveyor direction to be separated from the surface of the second conveyor belts  7  by a separator (not shown) and ejected into a paper ejector. 
         [0052]    If the first and second inkjet heads  2  and  3  need to be cleaned, as described above, the first and second cleaning caps  12  and  13  are moved upward to adhere the cap body to the nozzle side surface of the first and second inkjet heads  2  and  3 . In that state, a negative pressure is created in the cap body to suck up ink drops and bubbles through the nozzles of the first and second inkjet heads  2  and  3  so as to clean the first and second inkjet heads  2  and  3 . After the cleaning, the first and second cleaning caps  12  and  13  are moved downward. 
         [0053]    The inkjet printer includes a control unit that controls the printer itself. The control unit processes printing on a print medium by controlling a print unit or a paper feed unit based on print data input from a host computer  60  such as a personal computer or a digital camera, as shown in  FIG. 2 . The control unit includes: an input interface unit  61  for receiving print data input from the host computer  60 ; a control unit  62  comprising a microcomputer for executing print processing based on the print data input from the input interface unit  61 ; a gate roller motor driver  63  for controlling drive of the gate roller motor  17 ; a pickup roller motor driver  64  for controlling drive of a pickup roller motor  51  for driving the pickup roller  16 ; a head driver  65  for controlling drive of the inkjet heads  2  and  3 ; a right electric motor driver  66 R for controlling drive of the right electric motor  11 R; a left electric motor driver  66 L for controlling drive of the left electric motor  11 L; and an interface  67  for converting an output signal from each of the drivers  63  to  65 ,  66 R and  66 L into a drive signal used in the external gate roller motor  17 , the pickup roller motor  51 , the inkjet heads  2  and  3 , the right electric motor  11 R and the left electric motor  11 L and outputting the signal. 
         [0054]    The control unit  62  includes: a CPU (Central Processing Unit)  62   a  for executing various processing such as print processing; a RAM (Random Access Memory)  62   c  for temporally storing print data input via the input interface  61  or various data to execute processing such as printing of the print data, or for temporally deploying an application program such as for print processing; and a ROM (Read-Only Memory)  62   d  comprising a non-volatile semiconductor memory for storing a control program executed by the CPU  62   a . When the control unit  62  obtains print data (image data) from the host computer  60  via the interface unit  61 , the CPU  62   a  executes pre-determined processing on the print data, outputs print data (drive signal selection data SI&amp;SP) including which nozzle jets ink drops or how many ink drops are jetted, and outputs a control signal to each of the drivers  63  to  65 ,  66 R and  66 L based on the print data and input data from various sensors. When each of the drivers  63  to  65 ,  66 R and  66 L outputs the control signal, the interface unit  67  converts the signal into a drive signal, which causes the actuators corresponding to the plurality of nozzles of the inkjet heads, the gate roller motor  17 , the pickup roller motor  51 , the right electric motor  11 R, and the left electric motor  11 L to be individually actuated to execute paper feed and conveyor of the print medium  1 , posture control of the print medium  1 , and print processing onto the print medium  1 . Also, the control unit  62  outputs switch drive signals sw 1  and sw 2  to the low pass filter in a drive signal output circuit, which will be explained later, provided in the interface unit  67 , so that the low pass filter and the low-pass filter in the drive circuit including the actuators of nozzles through which ink drops are jetted remove only certain components or only the components within a predetermined range, so as to make drive signals actually applied to actuators uniform. The respective components of the control unit  62  are electrically connected to one another via a bus (not shown). 
         [0055]    Also, the control unit  62  outputs, in order to write waveform forming data DATA for forming a drive signal which will be described later into a waveform memory  701  which will be also described later, a write enable signal DEN, a write clock signal WCLK, and write address data A 0  to A 3  so that the 16-bit waveform forming data DATA is written into the waveform memory  701 . Further, the unit  62  outputs the following to the head driver  65 : read address data A 0  to A 3  to read out the waveform forming data DATA stored in the waveform memory  701 ; a first clock signal ACLK to set timing to latch the read-out waveform forming data DATA from the waveform memory  701 ; a second clock signal BCLK to set timing to add the latched waveform data; and a clear signal CLER to clear the latch data. 
         [0056]    The head driver  65  includes a drive waveform generator  70  for forming a drive waveform signal WCOM, and an oscillation circuit  71  for outputting a clock signal SCK. The drive waveform generator  70  includes, as shown in  FIG. 3 : the waveform memory  701  for storing waveform forming data DATA to generate a drive waveform signal input from the control unit  62  into a storage element corresponding to a pre-determined address; a latch circuit  702  for latching the waveform forming data DATA read out from the waveform memory  701  with the first clock signal ACLK described above; an adder  703  for adding an output of the latch circuit  702  and the waveform generation data WDATA output from a latch circuit  704  which will be described next; the latch circuit  704  for latching the added output by the adder  703  with the second clock signal BCLK described above; and a D/A converter  705  for converting the waveform generation data WDATA output from the latch circuit  704  into an analog signal. In this configuration, into the latch circuits  702  and  704  is input a clear signal CLER output from the control unit  62 , and when the clear signal CLER is turned off, the latch data is cleared. 
         [0057]    The waveform memory  701  has several bit memory elements arranged therein at each designated address in which addresses A 0  to A 3  and the waveform data DATA are stored, as shown in  FIG. 4 . Specifically, the clock signal WCLK and the waveform data DATA are input to the addresses A 0  to A 3  designated by the control unit  62 , and an input of the write enable signal DEN causes the waveform data DATA to be stored in the memory elements. 
         [0058]    Next, a principle of drive waveform signal generation by the drive waveform generator  70  will be described. First, waveform data which involves a voltage change amount of 0 per unit time is written at the address A 0  described above. Similarly, waveform data +ΔV 1  is written at the address A 1 , waveform data −ΔV 2  is written at the address A 2 , and waveform data +ΔV 3  is written at the address A 3 . The clear signal CLER clears data saved in the latch circuits  702  and  704 . The drive waveform signal WCOM rises to a midpoint potential (offset) according to the waveform data. 
         [0059]    In the above state, when the waveform data at the address A 1  is read and the first clock signal ACLK is input, the digital data +ΔV 1  is saved in the latch circuit  702 , as shown in  FIG. 5 . The saved digital data +ΔV 1  is input to the latch circuit  704  via the adder  703 . The latch circuit  704  saves output of the adder  703  in synchronization with a rise of the second clock signal BCLK. The output of the latch circuit  704  is also input to the adder  703 . Accordingly, the output of the latch circuit  704 , i.e., the drive signal COM is incremented by +ΔV 1  whenever the second clock signal BCLK rises. In this example, the waveform data at the address A 1  is read in a duration T 1 , and as a result, the signal COM is incremented until the digital data +ΔV 1  is tripled. 
         [0060]    Then, when the waveform data at the address A 0  is read and the first clock signal ACLK is input, digital data saved in the latch circuit  702  switches to 0. The digital data 0 goes through the adder  703  to be incremented whenever the second clock signal BCLK rises, similarly to the above description. However, since the digital data is 0, a previous value is substantially retained. In this example, the drive signal COM is retained at a certain value in a duration T 0 . 
         [0061]    Then, when the waveform data at the address A 2  is read and the first clock signal ACLK is input, digital data saved in the latch circuit  702  switches to −ΔV 2 . The digital data −ΔV 2  goes through the adder  703  to be incremented whenever the second clock signal BCLK rises, similarly to the above description. However, since the digital data is −ΔV 2 , the drive signal COM is substantially decremented by −ΔV 2  according to the second clock signal. In this example, the signal COM is decremented in a duration T 2  until the digital data −ΔV 2  becomes sixfold. 
         [0062]    When the digital signal generated in the above manner is converted into an analog signal by the D/A converter  705 , a drive waveform signal WCOM as shown in  FIG. 6  is gained. Then, a drive signal output circuit shown in  FIG. 7  amplifies the power of the analog signal and supplies the signal as a drive signal COM to the inkjet heads  2  and  3 , which can cause the actuators such as piezoelectric elements provided to the respective nozzles to be driven, so that each nozzle can jet ink drops. The drive signal output circuit is configured with: a modulator  24  for modulating a pulse of a drive waveform signal WCOM generated by the drive waveform generator  70 ; a digital power amplifier  25  for amplifying power of the modulated (PWM) signal subjected to the pulse modulation by the modulator  24 ; a low pass filter  26  for smoothing the modulated (PWM) signal subjected to the power amplification by the digital power amplifier  25 . 
         [0063]    A rise time of the drive signal COM corresponds to a stage in which the volume of a cavity (pressure chamber) communicating with a nozzle is increased to pull in ink (which may be expressed as pull in meniscus, from the viewpoint of the ink-jetted surface), while a fall time of the drive signal COM corresponds to a stage in which the volume of the cavity is decreased to push the ink out (which may be expressed as push out meniscus, from the viewpoint of the ink-jetted surface). As a result of the push-out of ink, the nozzle jets ink drops. A waveform of the drive signal COM or the drive waveform signal WCOM can be modified with waveform data 0, +ΔV 1 , −ΔV 2 , and +ΔV 3  written at the addresses A 0  to A 3 , the first clock signal ACLK, and the second clock signal BCLK, as can be readily inferred from the above description. 
         [0064]    A voltage gradient of a drive signal and a peak voltage of the drive signal COM in a voltage trapezoid wave may be variously changed, so that an amount and a speed of ink to be pulled in, and an amount and a speed of ink to be pushed out can be changed, which changes an amount of ink drops to be jetted so as to gain different sizes of ink dots. Thus, after a plurality of drive signals COM are sequentially connected in time to generate drive signals COM as shown in  FIG. 6 , a single drive signal COM may be selected from the signals to be supplied to the actuator  22  such as a piezoelectric element for one ejection of an ink drop, or a plurality of drive signals COM may be selected to be supplied to the actuators  22  such as piezoelectric elements for multiple ejections of ink drops, thereby various sizes of ink dots can be formed. That is, if a plurality of ink drops is dripped at the same position while the ink is not dried up, the same result can be substantially obtained as in the case where a large ink drop is jetted, and the size of an ink dot can be increased. Such a combination of techniques enables a multi-level tone to be accomplished. The drive pulse on the left end of  FIG. 6  only pulls in ink, but does not push out ink. This is called fine vibration which is used to inhibit or prevent a nozzle from being dried without ejection of ink drops. 
         [0065]    As a result, the following are input to the inkjet heads  2  and  3 : the drive signal COM generated by the drive signal output circuit; a drive signal selection data SI&amp;SP which selects a nozzle for ejection based on print data and determines a timing of connection to the drive signal COM of an actuator such as a piezoelectric element; a latch signal LAT and a channel signal CH which connects the drive signal COM and the actuators of the inkjet heads  2  and  3  based on the drive signal selection data SI&amp;SP after nozzle selection data is input to all of the nozzles; and a clock signal SCK which transmits the drive signal selection data SI&amp;SP as a serial signal to the inkjet heads  2  and  3 . Hereinafter, when a plurality of drive signals COM are sequentially connected in time to be output, a single drive signal COM is referred to as a drive pulse PCOM, and when the drive pulses PCOM are sequentially connected in time, the whole signals are referred to as a drive signal COM. 
         [0066]    Next, a structure to connect a drive signal COM output from the drive signal output circuit to the actuator such as a piezoelectric element will be described.  FIG. 8  is a block diagram of a selector for connecting a drive signal COM to an actuator such as a piezoelectric element. The selector is configured with: a shift register  211  for saving drive signal selection data SI&amp;SP to specify an actuator such as a piezoelectric element corresponding to a nozzle through which ink drops are jetted; a latch circuit  212  for temporarily saving data of the shift register  211 ; a level shifter  213  for converting a level of an output of the latch circuit  212 ; and a selection switch  201  for connecting a drive signal COM to an actuator such as a piezoelectric element in response to an output of the level shifter. 
         [0067]    To the shift register  211 , drive signal selection data SI&amp;SP are sequentially input, and also a storage area thereof is sequentially shifted from a first stage to a subsequent stage in response to an input pulse of a clock signal SCK. After drive signal selection data SI&amp;SP for the number of nozzles is stored in the shift register  211 , the latch circuit  212  latches each output signal of the shift register  211  according to an input latch signal LAT. The level of a signal saved in the latch circuit  212  is converted into a voltage level which enables a turning on/off of the selection switch  201  in a next stage by the level shifter  213 . This operation is required because the drive signal COM has a voltage higher than an output voltage of the latch circuit  212 , and accordingly the selection switch  201  is set to operate at a high operating voltage range. Thus, the actuator such as a piezoelectric element in which the selection switch  201  is closed by the level shifter  213  is connected to the drive signal COM at a timing to connect the drive signal selection data SI&amp;SP. After the drive signal selection data SI&amp;SP of the shift register  211  is saved in the latch circuit  212 , next print information is inputted to the shift register  211 , and data saved in the latch circuit  212  is sequentially updated at a timing to jet ink drops. Reference character HGND in the drawing denotes a ground terminal of the actuator such as a piezoelectric element. 
         [0068]    According to the selection switch  201 , an input voltage of the actuator  22  is maintained at the voltage just before the actuator such as a piezoelectric element is separated from the drive signal COM even after the separation. 
         [0069]      FIG. 9  shows a specific configuration between the modulator  24  of the drive signal output circuit and the low pass filter  26  described above. A general pulse width modulator (PWM) was used for the modulator  24  for modulating a pulse of a drive waveform signal WCOM. The pulse width modulator  24  is configured with a known triangular wave oscillator  32 , and a comparator  31  for comparing a triangular wave output from the triangular wave oscillator  32  and the drive waveform signal WCOM. According to the pulse width modulator  24 , as shown in  FIG. 10 , a modulated (PWM) signal Hi is output when the drive waveform signal WCOM is equal to a triangular wave or more, and a modulated (PWM) signal Lo is output when the drive waveform signal WCOM is smaller than a triangular wave. In the present embodiment, a pulse width modulator is used as a pulse modulator, but a pulse density modulator (PDM) may be employed instead. 
         [0070]    The digital power amplifier  25  is configured with a half bridge driver stage  33  including both a MOSFETTrP and a MOSFETTrN which substantially amplify power, and a gate drive circuit  34  for modifying the gate-source signals GP and GN of the MOSFETTrP and TrN based on a modulated (PWM) signal from the modulator  24 . The half bridge driver stage  33  is a push-pull combination of the high-side MOSFETTrP and the low-side MOSFETTrN.  FIG. 11  shows the changes of GP, GN and Va in response to a modulated (PWM) signal, where GP is gate-source signal of the high-side MOSFETTrP, GN is gate-source signal of the low-side MOSFETTrN, and Va is output of the half bridge driver stage  33 . The gate-source signals GP and GN of the MOSFETTrP and MOSFETTrN have a sufficient voltage value Vgs to turn ON the MOSFETTrP and MOSFETTrN, respectively. 
         [0071]    With a modulated (PWM) signal at Hi level, the gate-source signal GP of the high-side MOSFETTrP is at Hi level and the gate-source signal GN of the low-side MOSFETTrN is at Lo level. Thus, the high-side MOSFETTrP is turned into an ON state and the low-side MOSFETTrN is turned into an OFF state. As a result, the output Va from the half bridge driver stage  33  is turned to be a supply power VDD. Meanwhile, with a modulated (PWM) signal at Lo level, the gate-source signal GP of the high-side MOSFETTrP is at Lo level, and the gate-source signal GN of the low-side MOSFETTrN is at Hi level. Thus, the high-side MOSFETTrP is turned into an OFF state and the low-side MOSFETTrN is turned into an ON state. As a result, the output Va from the half bridge driver stage  33  becomes 0. 
         [0072]    The output Va from the half bridge driver stage  33  of the digital power amplifier  25  is supplied as a drive signal COM to the selection switch  201  via the low pass filter  26 . The low pass filter  26  is configured with a low-pass filter including a combination of one resistor R, one inductance L, and two capacitances C 1  and C 2 . The low pass filter  26  having the low-pass filter is designed to sufficiently attenuate a high-frequency component, i.e., an amplified digital signal component of an output Va from the half bridge driver stage  33  of the digital power amplifier  25 , and not to attenuate a drive signal component COM (or drive waveform component WCOM). Between the two capacitances C 1  and C 2  and a signal line of an amplified digital signal, switches SW 1  and SW 2  are interposed for connecting each of the capacitances C 1  and C 2  to the signal line, which are opened/closed by the switch drive signals sw 1  and sw 2  from the above described control unit  62  respectively. In the present embodiment, the first capacitance C 1  is larger than the second capacitance C 2 . 
         [0073]    As described above, when the MOSFETTrP and TrN of the digital power amplifier  25  are digitally driven, the MOSFETs operate as switch elements so that currents flow into the ON-state MOSFETs. However, a drain-source resistance value is very small, hence almost no power loss is generated. On the other hand, no current flows into the OFF-state MOSFETs, thereby no power loss is generated. Thus, the power loss of the digital power amplifier  25  is extremely small, as the result of that small MOSFETs can be used, and a cooling unit such as a cooling plate radiator can be eliminated. While a transistor is linearly driven at an efficiency of about 30%, a digital power amplifier can be driven at an efficiency of 90% or more. In addition, since one transistor requires a cooling plate radiator of 60 mm square, the elimination of such a cooling plate radiator provides a distinct advantage in an actual layout. 
         [0074]    Next, the switch drive signals sw 1  and sw 2  output from the control unit  62  will be described below. For example, when one actuator  22  such as a piezoelectric element is connected as shown in  FIG. 12A , the trapezoidal waveform of a drive pulse PCOM or drive signal COM is rounded off upon the connection of a plurality of actuators  22  such as piezoelectric elements ( 1440  nozzle of  FIG. 12A ). Actual measurements of the frequency characteristics of a drive circuit with actuators  22  such as piezoelectric elements demonstrate lower gains as a result of the increased number of the actuators  22  in connection. This is because the actuators  22  such as piezoelectric elements are connected in parallel by the above described selector. The actuator  22  such as a piezoelectric element has a capacitance Cn. For example, whenever an additional actuator  22  such as a piezoelectric element is connected to a resistor R and an inductance L of the low pass filter  26  shown in  FIG. 13A , the additional capacitance Cn of the actuator  22  such as a piezoelectric element is connected in parallel as shown in  FIGS. 13   b ,  13   c , and  13   d , resulting in that the whole drive circuit forms a low-pass filter. Needless to say, any drive signal COM or drive pulse PCOM is rounded off and supplied to the drive circuit which is a low-pass filter as a whole, without any high frequency component. 
         [0075]    In the present embodiment, the capacitances C 1  and C 2  provided in the low pass filter  26  are selectively connected to the drive circuit so as to limit the characteristics of the low-pass filter of the whole drive circuit to a certain amount or within a predetermined range, so that drive signals actually applied to actuators can be uniform. Specifically, a calculation processing shown in  FIG. 14  is performed in the control unit  62 , and switch drive signals sw 1  and sw 2  are generated and output based on the calculation result, and the capacitances C 1  and C 2  in the low pass filter  26  are appropriately connected. In the calculation processing, first, as Step S 1 , the number n of the actuators of nozzles for jetting ink drops (hereinafter, also referred to as the number of driving actuators) is calculated using drive signal selection data SI&amp;SP. 
         [0076]    Then, the processing goes to Step S 2 , where it is determined if the number n of driving actuators calculated at Step S 1  is equal to 0 or more up to a first predetermined value N 1  or not, and when the number n of driving actuators is equal to 0 or more up to a first predetermined value N 1 , the processing goes to Step S 3 , or otherwise the processing goes to Step S 4 . 
         [0077]    At Step S 4 , it is determined if the number n of driving actuators calculated at Step S 1  is above the first predetermined value N 1  and also equal to a second predetermined value N 2  or less which is larger than the first predetermined value N 1  or not, and when the number n of driving actuators is above the first predetermined value N 1  and also equal to the second predetermined value N 2  or less, the processing goes to Step S 5 , or otherwise the processing goes to Step S 6 . 
         [0078]    At Step S 6 , it is determined if the number n of driving actuators calculated at Step S 1  is above the second predetermined value N 2  and also equal to a third predetermined value N 3  or less which is larger than the second predetermined value N 2  or not, and when the number n of driving actuators is above the second predetermined value N 2  and also equal to the third predetermined value N 3  or less, the processing goes to Step S 7 , or otherwise the processing goes to Step S 8 . 
         [0079]    At Step S 3 , the first switch drive signal sw 1  is set to be ON, and the second switch drive signal sw 2  is set to be ON, and then the processing goes to Step S 9 . 
         [0080]    At Step S 5 , the first switch drive signal sw 1  is set to be OFF, and the second switch drive signal sw 2  is set to be ON, and then the processing goes to Step S 9 . 
         [0081]    At Step S 7 , the first switch drive signal sw 1  is set to be ON, and the second switch drive signal sw 2  is set to be OFF, and then the processing goes to Step S 9 . 
         [0082]    At Step S 8 , the first switch drive signal sw 1  is set to be OFF, and the second switch drive signal sw 2  is set to be OFF, and then the processing goes to Step S 9 . 
         [0083]    At Step S 9 , the first and second switch drive signals sw 1  and sw 2  are output, and then the processing returns to the main program. 
         [0084]    According to the calculation processing, when the number n of driving actuators, that is, the number of the actuators  22  such as piezoelectric elements which are connected to a drive signal COM (drive circuit) is equal to 0 or more up to a first predetermined value N 1 , the first capacitance C 1  and the second capacitance C 2  are connected to the drive circuit; when the number of the driving actuators  22  is above the first predetermined value N 1  and also equal to the second predetermined value N 2  or less, the second capacitance C 2  is connected to the drive circuit; when the number of driving actuators  22  is above the second predetermined value N 2  and also equal to the third predetermined value N 3  or less, the first capacitance C 1  is connected to the drive circuit; when the number of driving actuators  22  is above the third predetermined value N 3  (and equal to the maximum value N 4  or less), no capacitance is connected. As described above, and also as shown by the broken line of  FIG. 15 , since the capacitances of the drive circuit are increased as the number of the driving actuators  22  connected to the drive signal COM (drive circuit) is increased, the total capacitance C TOTAL  of the drive circuit of the present embodiment changes as shown by the solid line of  FIG. 15 . 
         [0085]    Therefore, in the present embodiment, the capacitances connected to the drive circuit are increased for the smaller number of the driving actuators  22 , so as to limit the capacitance of the whole drive circuit to a predetermined range, and then to limit the components removed by the low-pass filter in the drive circuit to a predetermined range, which makes the drive signals COM actually applied to the driving actuators  22  uniform. That is, in the present embodiment, the value of the capacitance which is connected to a drive circuit (drive signal COM) is changed depending on the number of the driving actuators  22 , which controls the frequency characteristics of the drive circuit itself and makes the drive signals COM actually applied to the actuators  22  uniform. Of course, when the number of the capacitances connectable in parallel to a drive circuit (drive signal COM) is increased and the connected capacitances are finely adjusted depending on the number of the driving actuators  22 , the capacitance can be held constant or almost constant for the whole drive circuit, which limits the components removed by the low-pass filter of the drive circuit to a certain amount and makes the drive signals actually applied to the actuators  22  uniform. In the present embodiment, since the low-pass filter in the drive circuit inevitably removes predetermined low frequency components of a drive signal COM, desirably the components are added to drive signals COM or drive waveform signals WCOM in advance. 
         [0086]    As described above, according to a head drive apparatus of an inkjet printer of the present embodiment, the drive waveform generator  70  generates a drive waveform signal WCOM as a reference of a signal for controlling the drive of an actuator such as a piezoelectric element, the generated drive waveform signal WCOM is pulse-modulated by the modulator  24  such as a pulse width modulator, the modulated signal which is pulse-modulated is power-amplified by the digital power amplifier  25 , and the amplified digital signal which is power-amplified is smoothed by the low pass filter  26  to be supplied to the actuator as a drive signal, thereby the low pass filter  26  has a filter characteristics to sufficiently smooth only the amplified digital signal component, which enables rapid rise and fall of a drive signal to an actuator, and eliminates a cooling unit such as a cooling plate radiator or the like because the digital power amplifier  25  having a high power-amplification efficiency efficiently amplifies the power of a drive signal. 
         [0087]    Also, the frequency characteristics of the low pass filter  26  is adjusted depending on the number of the actuators  22  of nozzles for jetting ink drops, thereby the low-pass filter in the drive circuit removes only certain components or only the components within a predetermined range, which makes drive signals COM actually applied to actuators  22  uniform. In addition, a plurality of capacitances C 1  and C 2  are provided which are connectable in parallel relative to amplified digital signals and switches SW 1  and SW 2  for individually connecting the capacitances C 1  and C 2  to the amplified digital signals, thereby the capacitances connected in parallel to amplified digital signals are increased for the smaller number of the actuators  22  of nozzles for jetting ink drops, thereby the low-pass filter in the drive circuit removes only certain components or only the components within a predetermined range, which makes drive signals actually applied to the actuators uniform. 
         [0088]      FIG. 16  shows another embodiment of a drive waveform generator and a modulation section included in a head drive apparatus of an inkjet printer according to the present invention. The drive waveform generator  70  of  FIG. 3  converts a digitally composed drive waveform signal into analog by the D/A converter  705 , and outputs the analog signal. To the contrary, in  FIG. 16 , the memory controller  41  reads out digital waveform data from the memory unit  42 , so that the read out digital waveform data is compared with the number value of the numerical value generator  43  which corresponds to a triangular wave at the comparing unit  44  to determine Hi and Lo of the modulated (PWM) signal, which is output as a modulated (PWM) signal. In this case, all the processes are digitally controlled up to the output of the modulated (PWM) signal, which allows the memory control unit  41 , the memory unit  42 , the numerical value generator  43 , and the comparing unit  44  to be cooperated in a CPU or a gate array. In this case, the memory controller  41  and the memory unit  42  correspond to drive waveform generator of the present invention, and the numerical value generator  43  and the comparing unit  44  form a modulation section. 
         [0089]      FIG. 17  shows another embodiment of the low pass filter  26 . In the embodiment, a variable capacitance Cv is used, and the control unit  62  outputs a control signal cvar to adjust the capacitance of the variable capacitance Cv. According to the embodiment, a capacitance of a low pass filter can be finely adjusted, thereby a low-pass filter in a drive circuit removes only certain components, which makes drive signals actually applied to actuators uniform or almost uniform. 
         [0090]    In the above described embodiments, only the example in which a head drive apparatus of an inkjet printer of the present invention is applied to a line head inkjet printer has been explained in detail, but a head drive apparatus of an inkjet printer of the present invention can be applied to any type of inkjet printer including a multi-pass printer.