Abstract:
A printing apparatus to which a printing material container is detachably mounted, the printing material container having a piezoelectric element for detecting the amount of material stored in the container and a memory unit storing the natural vibration frequency of the piezoelectric element, the printing apparatus comprising means for acquiring the frequency information from the memory unit, means for generating a drive having a first signal waveform at a first frequency and a second signal waveform at a second frequency different from the first frequency, means for selectively supplying either the first or second signal waveform to the piezoelectric element so as to increase the amplitude of the vibrations of the piezoelectric element; means for detecting a response signal, means for measuring the vibration frequency contained in the response signal, and means for determining the amount of material stored in the printing material container based on the measured vibration frequency.

Description:
BACKGROUND OF THE INVENTION 
   The entire disclosure of Japanese Patent Application No. 2006-115482, filed Apr. 19, 2006 is expressly incorporated herein by reference. 
   1. Technical Field 
   The present invention relates to a printing apparatus. More specifically, the present invention relates to a method of detecting the amount of a printing material in a printing material storage container. 
   2. Related Art 
   Many ink jet printing apparatuses contain a printing material storage container which includes a sensor for detecting the amount of remaining printing material in the container. One example of a sensor is a piezoelectric element which has the ability to expand and contract upon application of a voltage. The piezoelectric element oscillates upon application of the voltage and outputs an output signal. Thus, the printing apparatus applies the voltage to the piezoelectric element and measures the oscillation frequency of the piezoelectric element contained in the output signal to determine whether or not a predetermined amount of printing material remains in the printing material storage container. 
   Typically, the frequency of the voltage applied to the piezoelectric element is adjusted to be a resonant frequency of the sensor and the printing material stored in the printing material storage container, so that the amplitude of the oscillation of the piezoelectric element is increased and oscillation frequency measurement is more accurate. 
   Often, however, the sensors contain manufacturing errors generated during the manufacturing process. Often, the amplitude of the oscillation of the piezoelectric element may be reduced according to the manufacturing errors of the sensors, while the drive signal which is used to drive the sensors are constant. This makes the measurement of the oscillation frequency of the piezoelectric element difficult to measure with a high degree of accuracy, that the output signals outputted from the sensors may differ even though the same amount of printing material remains in the printing material storage container. Consequently, there is currently a problem accurately measuring the amount of printing material stored in the printing material storage container. 
   BRIEF SUMMARY OF THE INVENTION 
   In order to solve at least part of the problems shown above, one aspect of the invention provides a printing apparatus configured to measure the amount of the printing material stored in the printing material storage container. Further, one advantage of the invention is a more accurate measurement of the amount of printing material stored in a printing material storage container. 
   The printing apparatus of the invention comprises an acquiring unit capable of acquiring frequency information from a memory, a drive signal generating unit capable of generating and outputting a drive signal which may be used for driving a piezoelectric element which has a first signal waveform at a first frequency and a second signal waveform at a second frequency which is different from the first frequency, a supply unit capable of selecting a waveform which increases the amplitude of oscillations of the piezoelectric element from the first signal waveform and the second signal waveform of the outputted drive signal based on frequency information and supplying a selected drive signal having the selected signal waveform to the piezoelectric element, a detecting unit capable of detecting a response signal which is outputted in association with the oscillation of the piezoelectric element after having stopped the supply of the selected drive signal, a measuring unit configured to measure the oscillation frequency of the piezoelectric element included in the response signal, and a determining unit configured to determine the amount of the printing material stored in the printing material storing container on the basis of the oscillation frequency. 
   One advantage of the present invention is that the residual oscillation of the piezoelectric element is excited effectively using only one drive signal. Therefore, since it is no longer necessary to generate a drive signal for each printing material storage container, the processing load and processing time of the printing apparatus is reduced. Furthermore, the present invention is capable of detecting the response signal more accurately, resulting in a more accurate measurement of the amount of the printing material in the storage container. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
       FIG. 1  is an exemplary schematic configuration of a printing system. 
       FIG. 2  is an exemplary illustration of a main controller. 
       FIG. 3  is an explanatory drawing showing an electric configuration of a sub-controller and a cartridge according to the first example. 
       FIG. 4  is an explanatory drawing showing an example of a functional block of a switch controller according to the first example. 
       FIG. 5A  is an explanatory front view showing a configuration of an ink cartridge according to the first example. 
       FIG. 5B  is an explanatory side view showing the configuration of the ink cartridge according to the first example. 
       FIG. 6A  is an explanatory pattern cross-sectional view of a peripheral portion of a sensor provided on the ink cartridge when ink remains according to the first example. 
       FIG. 6B  is an explanatory pattern cross-sectional view of the peripheral portion of the sensor provided on the ink cartridge when ink does not remain according to the first example. 
       FIG. 7A  is an explanatory drawing showing an error range of the characteristic frequency of the cartridge when ink remains according to the first example. 
       FIG. 7B  is an explanatory drawing showing the error range of the characteristic frequency of the cartridge when ink does not remain according to the first example. 
       FIG. 8  is a waveform chart showing an example of a pulse waveform of a drive signal according to the first example. 
       FIG. 9  is an explanatory drawing showing an example of switch control data according to the first example. 
       FIG. 10  is a flowchart showing an ink amount determination process according to the first example. 
       FIG. 11  is a timing chart for explaining a frequency measurement process according to the first example. 
       FIG. 12  is a waveform chart showing an example of a pulse waveform of a drive signal according to a second example. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring now to the drawings, the invention will be described using a series of examples listed below. 
   A. First Example 
   A1. System Configuration 
     FIG. 1  is a schematic configuration of an exemplary printing system. The printing system includes a printer  20  and a computer  90 . The printer  20  is connected to the computer  90  via a connector  80 . 
   The printer  20  includes a secondary scan feeding mechanism, a main scan feeding mechanism, a head control mechanism, and a main controller  40  for controlling the respective mechanisms. The secondary scan feeding mechanism includes a paper feed motor  22  and a platen  26 . The secondary scan feeding mechanism transports paper P by transmitting the rotation of the paper feed motor to the platen in the secondary scanning direction. The primary scan feeding mechanism includes a carriage motor  32 , a pulley  38 , a drive belt  36  tightly extended between the carriage motor  32  and the pulley  38 , and a sliding shaft  34  placed parallel to the platen shaft  26 . The sliding shaft  34  holds a carriage which is fixed to the drive belt  36  in a manner that allows the carriage to slide along the sliding shaft  34 . The rotation of the carriage motor  32  is transmitted to the carriage  30  via the drive belt  36 . The carriage  30  moves reciprocally along the axial direction (primary scanning direction) of the platen  26  via the sliding shaft  34 . The head control mechanism includes a printing head unit  60  mounted to the carriage  30 . The head control mechanism causes the printing head  69  to discharge ink on the paper P. The printer  20  further includes an operating unit  70  which allows the user to select various settings and confirm the status of the printer. 
   The printing head unit  60  includes a print head  69  and a cartridge mounting portion. The cartridge mounting portion accommodates six ink cartridges  100   a  to  100   f . The printing head unit  60  further includes a sub-controller  50 . 
   The print head  69  includes a plurality of nozzles and a plurality of piezoelectric elements, and discharge ink drops from the respective nozzles according to the voltage applied to the respective piezoelectric elements to form dots on the paper P. 
   The ink cartridges  100   a  to  100   f  each are provided with a sensor which includes a piezoelectric element. The printer  20  supplies a drive signal to the piezoelectric elements of the sensors. The printer  20  determines the amount of ink stored in the ink cartridges by measuring the oscillation frequencies of the piezoelectric elements which is included in the response signals that are outputted from the piezoelectric elements, compared to the residual oscillations generated in the piezoelectric elements after the drive signal is stopped. Hereinafter, the ink cartridge is referred simply to as “cartridge.” 
   A2. Circuit Configuration of Printer 
   Referring now to  FIGS. 2-4 , a circuit configuration of the printer  20  will be described.  FIG. 2  is a drawing illustrating an exemplary electrical configuration of the main controller  40 .  FIG. 3  is a drawing illustrating an exemplary electric configuration of the sub-controller  50  and a cartridge.  FIG. 4  is a block diagram illustrating the switch controller. 
   The main controller  40  includes a CPU  41 , a memory  42 , an oscillator  43  configured to generate clock signals, an input and output unit (PIO)  44  configured to transmit signals between peripheral devices and transmit information to the sub-controller  50 , a drive signal generating circuit  46 , a drive buffer  47 , and an allotter  48 . These components are connected via buses  49 . The busses  49  are also connected to a connector  80 , and the main controller  40  is connected to the computer  90  via the busses  49  and the connector  80 . Within this configuration, the above-described components are capable of exchanging data. 
   The drive buffer  47  is used as a buffer for supplying dot ON and OFF signals to the print head  69 . The allotter  48  allots drive signals from the drive signal generating circuit  46  to the print head  69  at predetermined times. 
   The drive signal generating circuit  46  generates head drive signals PS, which are supplied to the print head  69  via the allotter  48 , along with drive signals DS which are supplied to the piezoelectric elements  112  via the sub-controller  50 . Hereinafter, the term “drive signal” is a “sensor drive signal.” The drive signal generating circuit  46  outputs the drive signal DS via the sub-controller  50 . The drive signal DS has a first signal waveform at a frequency F 1  and a second signal waveform at a frequency F 2  which is different from the frequency F 1 . In this example, the first signal waveform and the second signal waveform are generated so as to be arranged in series, and are outputted in sequence from the drive signal generating circuit  46 . 
   The CPU  41  acquires frequency information  135  (shown in  FIG. 3 ) stored in the memory  42  from the sub-controller  50 . 
   The CPU  41  generates a first switch control data SD 1  for selecting either the first signal waveform SP 1  or the second signal waveform SP 2 , based on the acquired frequency information  135 , and supplies a drive signal having only the selected signal waveform to the piezoelectric elements. Hereinafter, the drive signal having only the selected signal waveform is referred to as the “selected drive signal.” The CPU  41  sends the generated first switch control data SD 1  to the sub-controller  50 . The first switch control data SD 1  is data for controlling a first switch SW 1 . The CPU  41  generates second switch control data SD 2  for controlling a second switch SW 2  and third switch control data SD 3  for controlling a third switch and sends the same to the sub-controller  50 . The switch control data SD will be described in detail later. 
   The sub-controller  50  is a circuit for executing a process relating to the cartridges  100   a  to  100   f  in cooperation with the main controller  40 .  FIG. 3  the shows the portions of the circuit which are used during the ink measuring process. The sub-controller  50  is provided with a calculator  51 , the three switches SW 1  to SW 3 , and an amplifier  52 . 
   The calculator  51  includes a CPU  511 , a memory  513 , an interface (“I/F”)  514 , an I/O portion (“SIO”)  515  for transmitting signals between the components in the sub-controller  50  and the cartridges  100   a  to  100   f , and a switch controller  516 . The respective components of the main controller  40  are connected via basses  519 . The calculator  51  receives signals from the main controller  40  via the interface  514 . The calculator  51  controls the three switches SW 1  to SW 3  via the switch controller  516 . The calculator  51  transmits output from the amplifier  52  via the SIO  515 . 
   The switch controller  516  controls the first switch SW 1  to the third switch SW 3  according to the switch control data SD. The detailed functional blocks of the switch controller  516  will be described in reference to  FIG. 4 . 
   As shown in  FIG. 4 , the switch controller  516  includes a controller  210 , and switch control signal output circuits  220   a ,  220   b  and  220   c  which are configured for each switch. The switch control signal output circuit  220   a  is connected to the first switch SW 1  and controls the connecting state of the first switch SW 1 . The switch control signal output circuit  220   b  is connected to the second switch SW 2  and controls the connecting state of the second switch SW 2 . The switch control signal output circuit  220   c  is connected to the third switch SW 3  and controls the connecting state of the third switch SW 3 . Each of the switch control signal output circuits  220   a  to  220   c  include a shift register  200 , a latch circuit  201 , and a data decoder  202 . 
   Clock signals CLK, latch signals LAT, change signals CH, and switch control data SD are each supplied from the CPU  41  to the switch controller  516 . The switch control data SD is transferred to the shift register  200  synchronously with the clock signals CLK from the oscillator  43  of the main controller  40 . The transferred switch control data SD is latched once by the latch circuit  201 . The latched switch control data SD is entered to the data decoder  202 . 
   The controller  210  receives input of the latch signals LAT and the change signals CH. The controller  210  generates the switch control signal CS for ON and OFF, controlling the switch on the basis of the latch signals LAT and the change signal CH. The switch control signal CS which is generated by the controller  210  is supplied to the data decoder  202 . The data decoder  202  outputs the switch control signal CS to the switch on the basis of the latched switch control data SD. The switch control signal CS will be described in greater detail below. 
   The first switch SW 1  is a one-channel analog switch. One of the terminals of the first switch SW 1  is connected to the drive signal generating circuit  46  of the main controller  40 , and the other terminal is connected to the second switch SW 2  and the third switch SW 3 . The first switch SW 1  is set to the connected state while a selected drive signal SDS is supplied, and is set to the disconnected state when detecting a response signal RS from the sensor  110 . 
   The second switch SW 2  is a 6-channel analog switch. One of the terminals on one side of the second switch SW 2  is connected to the first switch SW 1  and the third switch SW 3 , and the six terminals on the other side are each connected to the electrodes of the sensors  110  of the six cartridges  100   a  to  100   f . The other electrode of each sensor  110  is grounded. The six cartridges  100   a  to  100   f  are selected in sequence by switching the second switch SW 2  in sequence. 
   The third switch SW 3  is a one-channel analog switch. One of the terminals of the third switch SW 3  is connected to the first switch SW 1  and the second switch SW 2 , and the other terminal is connected to the amplifier  52 . The third switch SW 3  is set to the disconnected state when supplying the drive signal DS to the sensor  110 , and is set to the connected state by receiving a supply of the ON signals from the switch controller  516  when detecting the response signal RS from the sensor  110 . 
   The amplifier  52  includes an OP amplifier, and functions as a comparator for comparing the response signal RS and a reference voltage Vref, and outputs high signals when the voltage of the response signal RS is the reference voltage Vref or higher and outputs low signals when the voltage of the response signal RS is lower than the reference voltage Vref. Therefore, output signals QC from the amplifier  52  are digital signals including only the high signals and the low signals. 
   The CPU  41  counts the output signals QC outputted from the amplifier  52 , measures the oscillation frequencies of the piezoelectric elements  112 , and determines the amount of ink stored in the ink cartridges based on the oscillation frequencies. Accordingly, the CPU  41  displays the result of on a display of the computer  90 , so that the user is notified of the ink amount. 
   A3. Detailed Configuration of Ink Cartridge and Sensor 
     FIGS. 5A-B  and  FIGS. 6A-B  illustrate a detailed configuration of the ink cartridge and the sensor.  FIGS. 5A and 5B  are a front view and side view of the ink cartridge.  FIGS. 6A and 6B  are cross-sectional views of a peripheral portion of the sensor located on the ink cartridge. 
   As shown in  FIG. 5A  and  FIG. 5B , a casing  102  of the cartridge  100   a  includes a plurality of storage chambers for storing ink. A main storage chamber MRM occupies a major portion of a capacity of the entire storage chamber. A first sub-storage chamber SRM 1  is in communication with an ink supply port  104 , which is located on its bottom surface. A second sub-storage chamber SRM 2  is also in communication with the main storage chamber MRM, and is located near the main storage chamber MRM&#39;s bottom surface. 
     FIGS. 6A and 6B  are cross-sectional views of a portion of the sensor taken along the line A-A in  FIG. 5B , as viewed from above. As shown in  FIGS. 6A and 6B , the sensor  110  includes a piezoelectric element  112  and a sensor attachment  113 . The piezoelectric element  112  includes a piezoelectric unit  114  and two electrodes  115 ,  116  on either side of the piezoelectric unit  114 , and is installed to the sensor attachment  113 . The piezoelectric unit  114  is a ferroelectric substance, and is formed of, for example, PZT (Pb(ZrxTi1-x)O3). Within the sensor attachment  113  a substantially angular C-shaped bridge flow channel BR is formed. A portion of the sensor attachment  113  between the bridge flow channel BR and the piezoelectric element  112  is comprised of a thin film. In this arrangement, a peripheral portion of the piezoelectric element  112  including the bridge flow channel BR oscillates with the piezoelectric element  112 . 
   The ink stored in the cartridge  100   a  flows as indicated by a solid arrow in  FIGS. 5A ,  5 B,  6 A, and  6 B. More specifically, the ink stored in the main storage chamber MRM flows from the bottom surface area into the second sub-storage chamber SRM 2 . The ink flowing into the second sub-storage chamber SRM 2  flows from a first side hole  76 , to the bridge flow channel BR of the sensor attachment  113 , through a second side hole  75 , and into the first sub-storage chamber SRM 1 . The ink flowed into the first sub-storage chamber SRM 1  passes through the ink supply port  104  and is supplied to the print head unit  60 . 
     FIG. 6A  shows the state wherein a predetermined amount of ink remains in the cartridge  100   a  (hereinafter referred to as “remaining ink”). As shown in  FIG. 6A , the term “remaining ink” represents the state wherein the ink is in the bridge flow channel BR. That is, the term “remaining ink” represents a state wherein ink exists at a position of the cartridge  100   a  where the sensor  110  is installed (ink detecting position), and the ink is in contact with a portion of the thin film sandwiched between the bridge flow channel BR and the piezoelectric element  112  (ink detecting area) of the sensor attachment  113 . 
   In contrast,  FIG. 6B  shows the state wherein the ink is less than the predetermined amount (hereinafter referred to as “no remaining ink”). The term “no remaining ink” represents the state wherein the ink is not in the bridge flow channel BR. That is, the term “no remaining ink” represents a state wherein the ink does not exist at the ink detecting position, and the ink is not in contact with the ink detecting area. 
   A4. Drive Signal 
   The drive signal with improved detection accuracy of the oscillation frequencies will now be described. As described above, the printer  20  determines the amounts of the ink stored in the cartridges by supplying the drive signal to the piezoelectric elements provided on the cartridges and measuring the frequencies of the response signals outputted from the piezoelectric elements. Therefore, it is desirable to increase the amplitude of the response signals in order to improve the detection accuracy of the oscillation frequencies. Further, it is preferable to adjust the frequency of the drive signal to be equal to characteristic frequencies of the piezoelectric elements  112  in order to improve the detection accuracy of the oscillation frequencies of the response signals. The piezoelectric elements resonate and output response signals with large amplitudes by supplying a drive signal having the same frequency as the characteristic frequencies of the piezoelectric elements to the piezoelectric elements. 
   However, difficulties arise as the cartridge sensor is subject to the manufacturing errors within the manufacturing process. Therefore, in general, the characteristic frequency fF when ink remains and the characteristic frequency fE when ink does not remain have margins of error with respect to a target characteristic frequencies H 1  and H 2 , respectively. This margin of error will be described using  FIGS. 7A and 7B .  FIGS. 7A  and  7 B are drawings showing an exemplary error range of the characteristic frequency of the cartridge.  FIG. 7A  shows an error range of the characteristic frequency of the piezoelectric element when there is remaining ink in the container, and  FIG. 7B  shows an error range of the characteristic frequency of the piezoelectric element when there is no ink remaining in the container. 
   As shown in  FIG. 7A , when there is ink remaining in the container, there is an error range ER 1  from HFmin (KHz) to HFmax (KHz). On the other hand, as shown in  FIG. 7B , when there is no remaining ink, there is an error range ER 2  from HEmin (KHz) to HEmax (KHz). As shown in the figures, there is a smaller range of oscillations included in the error range ER 2  than in the error range ER 1 . 
   The method of generating the response signal when ink does not remain will be described. When the frequency of the drive signal is set to the same frequency as the intermediate frequency Hm of the error range ER 1  and is supplied to the piezoelectric element, the characteristic frequency fE of the piezoelectric element of the cartridge is included within the accuracy range of Equation 1 shown below. Hereinafter, the range expressed by the Equation 1 is referred to as a detectable range DR.
 
(drive signal frequency  F *3)α%≦characteristic frequency  fE ≦(drive signal frequency  F* 3)+α%   Equation 1:
 
   In Equation 1, the value a is an allowable limit of error calculated on the basis of the manufacturing test in the manufacturing process, and is α=8 in this example. When the characteristic frequency fE of the cartridge to be processed is included in the detectable range DR (DRmin (KHz) to DRmax (KHz)), the residual oscillation of the piezoelectric element is effectively excited and hence the amplitude of the response signal may be amplified. However, in situations, such as those shown in  FIG. 7 , when the characteristic frequency fE of the cartridge to be processed is higher than DRmax (KHz) (the hatched range in  FIG. 7B ) the residual oscillation of the piezoelectric element is not effectively excited, and the detection accuracy of the response signal is lowered. 
   In order to adjust the frequency of the drive signal to be the same as the characteristic frequency of the piezoelectric element of the cartridge, it is necessary to generate different drive signals every time the ink amount determination process is performed, requiring significant process time. 
   In order to solve this problem, the printer according to the invention generates and outputs a drive signal including two types of signal waveforms, SP 1  and SP 2 , each having different frequencies. The printer controls the connecting state of the first switch SW 1 , selects the signal waveform having a frequency closer to the characteristic frequency of the piezoelectric element from between SP 1  and SP 2 , and supplies a drive signal associated with the selected signal waveform to the piezoelectric element. Accordingly, it is not necessary to generate drive signals with differing frequencies for each cartridge to be processed, meaning that a drive signal capable of effectively exciting the residual oscillations of the piezoelectric elements is supplied. 
   In this example, a waveform of a given frequency F 1 , which is included in the error range ER 1  and is a frequency higher than the intermediate frequency Hm of the error range ER 1  is determined to be the first signal waveform SPA, and the waveform of a given frequency F 2 , which is included in the error range ER 1  and is a frequency lower than the intermediate frequency Hm of the error range ER 1  is determined to be the second signal waveform SP 2 . 
   Referring now to  FIG. 8 , the drive signal DS generated by the drive signal generating circuit  46  will be described.  FIG. 8  is a waveform chart showing an outputted drive signal and the selected drive signal SDS to be applied to the piezoelectric elements. 
   The CPU  41  issues instructions in order to generate the drive signal to the drive signal generating circuit  46  using a drive signal generating parameter stored in the memory  42 . The drive signal generating circuit  46  generates the drive signal DS according to the instructions in order to generate a drive signal, which is then issued from the CPU  41 . The drive signal generating parameter includes various parameters required for generating drive signal such as a drive voltage Vh, a maximum voltage VH, a minimum voltage VL, a ratio for defining the relation between the drive voltage Vh and the reference voltage Vref, the frequency F 1 , and the frequency F 2 . 
   The drive signal DS includes the first signal waveform SP 1  generated during a term Ta and the second signal waveform SP 2  generated during a term Tb of a drive signal cycle T. The term Ta is one cycle of the first signal waveform SP 1  and follows the equation Ta=1/F 1 . The term Tb is one cycle of the second signal waveform SP 2  and follows the equation Tb=1/F 2 . The drive signal cycle T (term Ta+term Tb) corresponds to one cycle T of the drive signal DS. 
   The method of selecting the drive signal waveform of the selected drive signal to be supplied to the piezoelectric element from the first signal waveform SP 1  and the second signal waveform SP 2  will now be described. The drive signal selecting process is executed by the CPU  41 . The characteristic frequency fF is calculated from the error range ER 1 , the error range ER 2 , and the characteristic frequency fE, using Equation 2 shown below. The characteristic frequency fE when there is no remaining ink is obtained through a test measurement during the manufacturing process.
 
 fF =( fE−HE min)*( HF max− HF min)/( HE max− HE min)+ HF min   Equation 2:
 
   The memory  130  includes the characteristic frequency fE of the piezoelectric element when there is no remaining ink, which is stored in advance as frequency information  135 . The CPU  41  acquires the characteristic frequency fE from the memory  130  of the cartridge to be processed via the sub-controller  50 , and calculates the characteristic frequency fF using Equation 2. When the calculated characteristic frequency fF is higher than the intermediate frequency Hm, the CPU  41  selects the first signal waveform SP 1  as a waveform of the selected drive signal, and when the calculated characteristic frequency fF is lower than the intermediate frequency Hm, the CPU  41  selects the second signal waveform SP 2  as a waveform of the selected drive signal. 
   When the selected drive signal comprising the first signal waveform SP 1  is supplied to the piezoelectric element, the detectable range DR is calculated using Equation 1. When the characteristic frequency fE of the piezoelectric element of the cartridge when there is no remaining ink is included in the detectable range DR of Equation 1, the residual oscillation of the piezoelectric element is effective. When the characteristic frequency fF when there is ink remaining in the cartridge, is included within the range of “drive signal frequency F±25%”, the residual oscillation of the piezoelectric element is effectively excited. 
   A5. Switch Control Data 
   The CPU  41  generates the first switch control data SD 1  on using the selection process shown above. Referring now to  FIG. 9 , the first switch control data SD 1  will be described.  FIG. 9  is an explanatory illustration showing the selection patterns of the selected drive signal and the first switch control data SD 1 . The selection table  500  shown in  FIG. 9  shows selected patterns of the signal waveform together with an association function between the first switch control data SD 1  and the characteristic frequency fF. For example, the CPU  41  selects (shown as “0”) the first signal waveform SP 1  as the waveform of the selected drive signal in the case where the characteristic frequency fF&gt;intermediate frequency Hm. In this case, as shown in  FIG. 9 , since the first switch control data SD 1  is [10], the CPU  41  generates the first switch control data SD 1 [10]. On the other hand, when the characteristic frequency fF≦intermediate frequency Hm, the second signal waveform SP 2  is selected as the waveform of the selected drive signal. In this case, since the first switch control data SD 1  is [01], the CPU  41  generates the first switch control data SD 1 [01] and sends the same to the calculator  51 . 
   A6. Switch Control Signal 
   The calculator  51  outputs the first switch control signal CS, which controls the connecting state of the first switch SW 1  according to the first switch control data SD 1  sent from the CPU  41 . The waveforms of the switch control signals and the selected drive signals to be applied to the piezoelectric elements will be described in reference to  FIG. 8 . The selected drive signals shown in  FIG. 8  indicate the drive signals to be applied to the piezoelectric elements. 
   The switch controller  516  outputs the switch control signal CS for controlling ON and OFF of the first switch SW 1  on the basis of the latch signal LAT, the change signal CH, and the first switch control data SD 1  supplied from the CPU  41 . When the switch control signal CS is at a high level, the first switch SW 1  is in the connected state. Therefore, as shown in  FIG. 8 , when the first switch control data SD 1  is [10], the switch controller  516  outputs high-level signals (ON signals) over the term Ta, and the first switch SW 1  is in the connected state. In contrast, when the switch controller  516  outputs low-level signals over the term Tb, the first switch SW 1  is in the disconnected state. Therefore, as shown in the selected drive signal SDS 1  in  FIG. 8 , only the signals having the first signal waveform SP 1  are supplied to the piezoelectric elements  112 . When the first switch control data SD 1  is [01], since the switch controller  516  outputs low-level signals over the term Ta, the switch is in the disconnected state, and the switch controller  516  outputs high-level signals over the term Tb, and the switch is in the connected state. Therefore, as shown in the selected drive signal SDS 2  in  FIG. 8 , only the signals having the second signal waveform SP 2  are supplied to the piezoelectric elements  112 . Accordingly, a drive signal DS which excites the piezoelectric elements  112  effectively is selected from the two signal waveforms SP 1  and SP 2 . 
   A7. Ink Amount Determination Process: 
   Referring now to  FIGS. 10 and 11 , the ink amount determination process that the main controller  40  and the sub-controller  50  of the printer  20  execute in cooperation will be described.  FIG. 10  is a flowchart explaining the ink amount determination process.  FIG. 11  is a timing chart for explaining a frequency measuring process. 
   The process of determining the ink amount is a process for determining whether the ink amount stored in the cartridge is more or less than a predetermined amount for each cartridge. The process of determining the ink amount is typically executed when the power of the printer  20  is turned ON. 
   The CPU  41  of the main controller  40  selects a cartridge as a target of the process of determining the ink amount from among the six cartridges  100   a  to  100   f  when the process is started (Step S 101 ). 
   The main controller  40  acquires the frequency information  135  relating to the characteristic frequency of the piezoelectric element  112  from the memory  130  provided on the target cartridge (Step S 102 ). More specifically, the main controller  40  sends a command for causing the sub-controller  50  to acquire the frequency information  135  stored in the memory  130  of the cartridge, in order to send the information to the calculator  51  of the sub-controller  50 . The CPU  511  of the calculator  51  acquires the frequency information  135  and sends the acquired frequency information  135  to the sub-controller  50 . 
   The main controller  40  generates the switch control data for determining the first switch control data SD 1  on the basis of the acquired frequency information  135  (Step S 103 ), using the process described above. In this example, the second signal waveform SP 2  is selected, and the first switch control data SD 1 [01] is generated. 
   The main controller  40  generates the drive signal DS having the first signal waveform SP 1  and the second signal waveform SP 2  and outputs the same to the piezoelectric element in order to execute the frequency measuring process (Step S 105 ). Referring now to a timing chart shown in  FIG. 11 , the frequency measuring process will be described. The clock signal CLK, a measurement command CM, the latch signal LAT, and the change signal CH shown in  FIG. 11  are signals that may be sent to the calculator  51  of the sub-controller  50  from the main controller  40  in the frequency measuring process. The switch control signal CS is a signal outputted from the switch controller  516 . The measurement command CM includes information for specifying the cartridge to be processed together with a command that instructs execution of the frequency measurement process. The drive signal DS is a signal outputted from the drive signal generating circuit  46  of the main controller  40  as described above. The response signal RS is a signal generated in association with the residual oscillation of the piezoelectric element after having supplied the drive signal DS. 
   The calculator  51  of the sub-controller  50  controls the second switch SW 2  according to the measurement command CM which the calculator  51  has received in advance to the timing when the latch pulse P 1  of the latch signal was received, and brings the piezoelectric element  112  of the cartridge to be processed into the state of being connected with the sub-controller  50 . Furthermore, the calculator  51  controls the connecting state of the first switch SW 1  on the basis of the first data of the first switch control data SD 1  at the time when the latch pulse P 2  is received. In this example, the first switch control data SD 1 [01] is supplied to the switch controller  516 . Since the first data of the first switch control data SD 1  is [0], the ON signal is not outputted to the first switch SW 1  from the switch controller  516 , and hence the first switch SW 1  is in the disconnected state. Furthermore, The calculator  51  brings the third switch SW 3  into the disconnected state at a timing when the latch pulse P 1  is received. Accordingly, the amplifier  52  is electrically disconnected from the drive signal generating circuit  46  and the piezoelectric element  112 , and hence the drive signal DS is not applied to the amplifier  52 . 
   The main controller  40  generates a change pulse P 2  of the change signal at a timing when the term Ta terminates. The calculator  51  controls the connected state of the first switch SW 1  based on the second data of the first switch control data SD 1  at the time when the change pulse P 2  is received. In this example, since the second data of the first switch control data SD 1  is [1], the ON signal is outputted from the switch controller  516  to the first switch SW 1 . The first switch SW 1  is set to the connected state upon reception of the ON signal. Accordingly, only the selected drive signal having the second signal waveform SP 2  is applied to the piezoelectric element  112 . 
   The main controller  40  generates a change pulse P 3  at the time when the application of the drive signal is terminated. The calculator  51  of the sub-controller  50  brings the first switch SW 1  into the disconnected state at the time when the change pulse P 3  is received. A term from the latch pulse P 1  to the change pulse P 3  is referred to as the drive voltage application term T 1 . 
   After having terminated the drive voltage application term T 1 , the piezoelectric element  112  is oscillated by the drive signal. The piezoelectric element  112  outputs a response signal RS according to distortion in association with the oscillation. After having generated the change pulse P 3 , the main controller  40  generates a change pulse P 4 . The calculator  51  of the sub-controller  50  brings the third switch SW 3  into the connected state at upon reception of the change pulse P 4 . Consequently, the response signal RS from the piezoelectric element  112  is supplied to the amplifier  52 . 
   The amplifier  52  functions as a comparator as described above, and outputs the output signal QC as a digital signal according to the waveform of the response signal RS to the calculator  51 . The calculator  51  calculates an oscillation frequency H of the response signal RS on the basis of the acquired output signal QC and sends the signal RS to the main controller  40 . 
   The main controller  40  determines the amount of ink in the cartridge based on the oscillation frequency H (Step S 105 ). Next, the main controller  40  determines if the amount of ink in the cartridge to be more than the predetermined amount when the oscillation frequency H is compared to the above-described characteristic frequency H 1  (Step S 106 ). Similarly, the main controller  40  determines if the amount of ink in the cartridge is smaller than the predetermined amount when the oscillation frequency H is compared to the characteristic frequency H 2  (Step S 107 ). 
   The main controller  40  sends the result of determination of the ink amount to the computer  90 . Accordingly, the computer  90  may notify the result of determination of the received ink amount to the user. 
   In the printing system of this invention, the drive signal has a plurality of signal waveforms with different frequencies. The plurality of signal waveforms are outputted and one is selected to form a drive signal according to the characteristic frequency of each ink cartridge, so that a selected drive signal includes only the selected signal waveform is supplied to the piezoelectric element. Therefore, in the ink amount determination process, it is no longer necessary to regenerate the drive signal for each cartridge, alleviating the processing load of the printing apparatus, and reducing the processing time of the process. 
   Since it is not necessary to configure a circuit individually for each signal waveform in order to generate the plurality of signal waveforms, the circuit required to execute the process of determining the amount of ink is simplified. 
   In accordance with one embodiment of the invention, the drive signal which is used to oscillate the piezoelectric element is selected from a first signal waveform SP 1  and a second signal waveform SP 2 , there is improved accuracy in detecting the response signal, and the accuracy of the ink amount determination is improved. 
   B. Second Example 
   In the example described above, one shot (one cycle) each of the first signal waveform SP 1  and the second signal waveform SP 2  are included in one cycle of the drive signal DS. In the second example, for example, two shots (two cycles) each of the signal waveforms may be included. 
   B1. Waveform of Drive Signal 
     FIG. 12  is a waveform chart showing a drive signal DS′ according to the second example. The drive signal DS′ is a signal outputted from the drive signal generating circuit  46 . Within the drive signal generating circuit  46 , a first signal waveform SP 1 ′ and a second signal waveform SP 2 ′ containing the waveforms for two cycles respectively are included in the drive signal cycle T of the drive signal DS′ as shown in  FIG. 12 . The term Ta indicates one cycle T of the signal at the frequency F 1 , and the term Tb indicates one cycle of the signal at the frequency F 2 . 
   In this example, when the first signal waveform SP 1  is selected as a waveform of the selected drive signal, only the first signal waveform SP 1 ′ including the waveforms for two cycles is supplied to the piezoelectric element, and the second signal waveform SP 2 ′ is not supplied to the piezoelectric element. 
   The piezoelectric element is excited in order to create a residual oscillation with a large amplitude in association with the increase in number of cycles (number of shots) of the waveform, resulting in improved detection accuracy of the response signal. However, this example results in increased processing time, in association with increase in number of shots of the waveform to be supplied to the piezoelectric element. Therefore, the waveform of the selected drive signal is preferably two shots or smaller. Accordingly, the amplitude of the oscillation of the piezoelectric element  112  is increased, the detection accuracy of the response signal is further improved, and the processing time is reduced. 
   As shown in  FIG. 12 , the numbers of shots included in the first signal waveform SP 1 ′ and the second signal waveform SP 2 ′ are preferably the same. This allows response signals of the same level to be detected at a high degree of accuracy irrespective of which one of the first signal waveform SP 1 ′ and the second signal waveform SP 2 ′ is supplied to the piezoelectric element. 
   C. Modification 
   In the examples described above, the drive signal having the waveforms at the two different frequencies are generated from within the error range ER 1  of the characteristic frequency when ink remains in the cartridge. However, it is also possible to generate the drive signal having a waveform of the drive signal for executing the ink amount determination process both when there is ink remaining in the cartridge and when there is no ink remaining in the cartridge. In this configuration, it is not necessary to regenerate the drive signal during the ink amount determination processes when ink remains in the cartridge and when ink does not remain in the cartridge, and hence the processing time may be preferably reduced. Since it is not necessary to configure the circuit for generating the drive signal to be used when executing the each of the processes for determining the ink amount, the circuit size may be reduced. 
   Although various examples of the invention have been described thus far, the invention is not limited to the examples shown above and, needless to say, various configurations may be employed without departing the scope of the invention.