Abstract:
Conventional drivers for transducers oftentimes did not provide an efficient driving mechanism because the driving signal was not “close enough” to the natural frequency of the transducer. Here, a driver for a transducer is provided that measures the natural frequency of the transducer and generates a driving signal accordingly. Thus, a more efficient driver is provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is claims priority to Japanese Patent Appl. No. 2008-284095 filed on Nov. 5, 2008, which is hereby incorporated by reference for all purposes. 
     TECHNICAL FIELD 
     The invention relates generally to piezoelectric driver and, more particularly, to a driver for a piezoelectric fan. 
     BACKGROUND 
     In recent years, there has been demand for high performance portable devices that are also aesthetically pleasing. Consequently, because electronics (including integrated circuit or ICs) have been confined to small housings, heat dissipation for these ICs has become an issue. Conventional fans used in computers and other applications generally use impellers to generate air flow for heat dissipation, but forced air impeller fans are impractical for compact, high performance devices due to size, power constraints, and noise. Therefore, small fans using piezoelectric elements have been studied as replacements for conventional forced air impeller fans. 
     Turning to  FIGS. 1A through 1D , a piezoelectric fan  100  can be seen. Fan  100  is generally comprised of a piezoelectric element  104  that is secured to a generally rectangular metal sheet  102  and which is driven by alternating current (AC) source  106 . As can be seen, piezoelectric element  104  is secured to one end of  102 , leaving the opposite end free. When driven by a sine wave from AC source  106  shown in  FIG. 1D , the free end of the sheet  102  vibrates (as shown in  FIG. 1C ). As shown, a piezoelectric fan is structurally simple, which smaller, consumes less power, and is quieter than convention forced air impeller fans. 
     Fan  100  has a natural frequency or mechanical resonance frequency based on physical characteristics. When an AC voltage from source  106  at a frequency equal to the natural frequency of fan  100  is supplied, the vibrational amplitude of sheet  102  is at a maximum, and when the frequency of the AC voltage from source  106  is a slightly different from the natural frequency of sheet  102 , the vibrational amplitude falls off drastically. Therefore, to drive the fan at a high efficiency, the frequency of the AC voltage from source  106  should match the natural frequency of the fan  100 . 
     SUMMARY 
     From a first viewpoint, the present invention provides a driver characterized by the fact that the driver is for driving an element that receives electric energy and generates mechanical or electrical vibration, and it has the following parts: a driving part that supplies a driving signal at a preset period or frequency to the element in a driving mode, and that stops the supply of the driving signal to the element in a measurement mode, a detecting part that detects the period or frequency of the free vibration generated in the element in the measurement mode, and a control part that alternately switches the operating mode between the driving mode and the measurement mode, and simultaneously, adjusts the period or frequency of the driving signal in the driving mode so that it approaches the detection result of the detecting part in the measurement mode. 
     The driving mode in which the driving signal is supplied to the element and the measurement mode in which the supply of the driving signal is stopped and the period of the free vibration generated in the element is detected are performed alternately and repeatedly by the driver, and the period of the driving signal in the driving mode is adjusted so that it is near the detection result of the free vibration period of the element in the measurement mode. 
     As a preferable scheme, the detecting part detects the period or frequency of the electric signal generated by the element in the free vibration. 
     As a preferable scheme, the driving part takes the output of the driving signal as a high impedance state in the measurement mode, and the detecting part inputs the electric signal to the element via the supply line of the driving signal. 
     As a preferable scheme, the control part sets the period or frequency of the driving signal in the driving mode based on the detection result of the detecting part in the measurement mode just before the driving mode, or based on the detection result of the detecting part in the measurement mode performed as a series of plural repeated rounds just before the driving mode. 
     As a preferable scheme, when the difference between the setting value of the period or frequency of the driving signal in the driving mode and the detection value of the detecting part in the measurement mode just after the driving mode exceeds a first threshold, the control part switches the operating mode to the measurement mode after output of the driving signal of N cycles (N is an integer of 1 or larger) in the driving mode. 
     As a preferable scheme, when the difference is below the first threshold, the control part switches the operating mode to the measurement mode after output of the driving signal of a predetermined number of cycles larger than the N cycles in the driving mode, or switches the operating mode to the measurement mode after output of the driving signal for a predetermined time longer than the N cycle period in the driving mode, or locks the operating mode in the driving mode. 
     As a preferable scheme, the control part outputs a signal indicating abnormal vibration of the element when the driving mode that outputs the N cycles of driving signal is repeated continuously for a predetermined number of rounds or a predetermined time. 
     As a preferable scheme, the control part operates as follows: when the detection value is outside a predetermined range, the period or frequency of the driving signal in the next the driving mode is set at a predetermined lower threshold or upper threshold; then, during the period until the difference falls below the first threshold, if the detection value is outside the predetermined range, the period or frequency of the driving signal in the next driving mode is changed by a predetermined increment or decrement with respect to the period or frequency of the driving signal in the driving mode in the last round. 
     As another scheme that may be adopted, the control part operates as follows: if the difference is over a second threshold larger than the first threshold, the period or frequency of the driving signal in the next the driving mode is set at a predetermined lower threshold or upper threshold; then, during the period until the difference falls below the first threshold, if the difference is over the second threshold, the period or frequency of the driving signal in the next the driving mode is changed by a predetermined increment or decrement with respect to the period or frequency of the driving signal in the driving mode of the last round. 
     As a preferable scheme, at the start of the driving mode, the control part matches the phase of the driving signal at the start to a phase that can be held by the driving signal in the last round of the driving mode. 
     As a preferable scheme, the driver has a binary value formation part that converts the electric signal to a binary signal. Here, the detecting part detects the time interval between the centers of two consecutive pulses in the binary signal. 
     According to a second viewpoint, the present invention provides a driving method characterized by the fact that the method is for driving an element that receives electric energy and generates mechanical or electrical vibration, and it has the following steps of operation: a first step of operation in which a driving signal at a predetermined period or frequency is supplied to the element during a predetermined period, a second step of operation in which the period or frequency of the electric signal generated by the free vibration of the element is measured, a third step of operation to determine whether the period or frequency of the electric signal is contained in a predetermined range, a fourth step of operation in which the following operation is executed: when the period or frequency of the electric signal is not contained in the predetermined range, a predetermined value is added to the period or frequency of the driving signal to set a new period or frequency, and at the same time, a first count value is counted up, a fifth step of operation in which the first count value is compared to a first value, a sixth step of operation in which the operation returns to the first step of operation if the first count value is smaller than the first value, a seventh step of operation in which the following operation is executed: when the period or frequency of the electric signal is within the predetermined range, the difference between the period or frequency of the driving signal and the period or frequency of the electric signal is compared to a predetermined value, and an eighth step of operation in which the following operation is executed: if the difference is smaller than the predetermined value, the driving signal is supplied to the element for a period longer than the predetermined period. 
     As a preferable scheme, in the eighth step of operation, if the difference is smaller than the predetermined value, the period or frequency of the electric signal is set at the new period or frequency of the driving signal, and at the same time, the driving signal is supplied to the element for a period longer than the predetermined period. 
     As a preferable scheme, a ninth step of operation also exists in which the following operation is executed: if the first count value matches the first value, a signal indicating abnormality of the element is output. 
     In addition, a tenth step of operation may also exist in which the following operation is executed: if the difference is not smaller than the predetermined value, the period or frequency of the electric signal is set as the new period or frequency of the driving signal, and the fifth step of operation is executed after the tenth step of operation. 
     As a preferable scheme, in the eighth step of operation, when the difference is smaller than the predetermined value repeatedly for plural successive rounds, the driving signal is fed to the element for a period longer than the predetermined period. 
     In addition, the second step of operation is entered after the eighth step of operation. 
     In accordance with another preferred embodiment of the present invention, 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and the specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1A  is a plan view of a piezoelectric fan; 
         FIG. 1B  is an elevation view of the piezoelectric fan of  FIG. 1A ; 
         FIG. 1C  is a plan view of the piezoelectric fan of  FIG. 1A  in operation; 
         FIG. 1D  is a diagram of the voltage versus time for the alternating current (AC) of the piezoelectric fan of  FIG. 1A ; 
         FIG. 2  is a diagram of an example of a driver for a piezoelectric fan in accordance with a preferred embodiment of the present invention; 
         FIG. 3  is a timing diagram for the operation of the period detector of the driver of  FIG. 2 ; 
         FIG. 4  is a flow chart illustrating the operation of the driver of  FIG. 2 ; 
         FIG. 5  is a timing diagram for the operation of the driver of  FIG. 2 ; and 
         FIG. 6  is a diagram illustrating the phase of the driving signal at the start of the driving mode. 
     
    
    
     DETAILED DESCRIPTION 
     Refer now to the drawings wherein depicted elements are, for the sake of clarity, not necessarily shown to scale and wherein like or similar elements are designated by the same reference numeral through the several views. 
     According to the present invention, the period or frequency of the free vibration of the element that takes place when supply of the driving signal is stopped is detected, and corresponding to the detection result, the period or frequency of the driving signal is adjusted so that the element is driven at a high efficiency at a frequency similar to the characteristic vibration frequency. 
     Referring to  FIG. 2  of the drawings, the reference numeral  200  generally depicts an example of a driver for a piezoelectric fan in accordance with a preferred embodiment of the present invention. As can be seen in  FIG. 2 , driver  200  provides energy to piezoelectric transducer PZT so as to drive it. For example, transducer PZT may be a piezoelectric fan  100 . Driver  200  generally comprises transistors Q 1 , Q 2 , and Q 3  (which are preferably a PMOS transistor, an NMOS transistor, and an NMOS transistor, respectively), resistor R, diodes D 1  and D 2 , comparator  206 , period detector  208 , and controller  210 . Controller  208  is generally comprised of edge detector  218 , registers  220 ,  226 , and  232 , flip-flops  242  and  238 , counters  240  and  236 , comparators  224  and  222 , adder  228 , frequency divider  234 , multiplexer or mux  230 , and sequence controller  244 . 
     As shown, transistors Q 1  and Q 2  are coupled in series with one another between power supply VDD and ground to drive the resistor R and transducer PZT. The drains of transistors Q 1  and Q 2  are commonly coupled to output node N 1  and are driven at their respective gates by gate driver  202 . Gate driver  202  drives the gates of transistors Q 1  and Q 2  corresponding to drive and mode signals DRIVE and MODE supplied by controller  210 , so that transistors Q 1  and Q 2  are alternately turned on and off. The switching of these transistors generates a driving signal that is output from the output node N 1 . Additionally, resistor R and transducer PZT are coupled in series between output node N 1  and ground, where resistor R and the parasitic electrostatic capacitance of transducer PZT form a low-pass that suppresses generation of audible sound that accompanies vibration of transducer PZT when driven. 
     In operation, however, gate driver operates in one of two operation modes (driving mode and measurement mode), which are indicated by mode signal MODE. When mode signal MODE is logic low, gate driver  202  operates in driving mode. In driving mode, gate driver  202  alternately turns “on” transistors Q 1  and Q 2  based one drive signal DRIVE to that the transducer PZT operates (i.e., as a fan). When mode signal MODE is logic high, gate driver  202  operates in measurement mode. In measurement mode, gate driver  202  turns “off” both transistors Q 1  and Q 2  so that output node N 1  passes into the high impedance state so that transducer PZT can freely vibrate. 
     Transistor Q 3  is also coupled to node N 1 , inserted into the signal path between node N 1  and the positive input terminal of comparator  206 . The gate of transistor Q 3  is coupled to controller  210  and is controlled by mode signal MODE so that transistor Q 3  is “off” in the driving mode and “on” in the measurement mode. During the measurement mode, comparator  206  compares the signal input from transducer PZT via transistor Q 3  to ground, outputting binary signal corresponding to the comparison result. Additionally, diodes D 1  and D 2  are coupled in parallel (with opposite polarity) between the input terminals of comparator  206  so as to restrict the amplitude of the input signal to comparator  206 . 
     In the measurement mode and based on the binary output of comparator  206 , the period detector  208  can determine the period of clock signal CLK. The period detector  208  is generally comprised of a divider  212  (which is a divide-by-2 divider), multiplexer or mux  210  and counter  216 . In the measurement mode, controller  210  provides a control signal EN to enable the counter  216 . The free vibration of transducer generates a signal that is converted to the binary signal seen in  FIG. 3 . During the logic high periods of the first pulse (P 1 ) and the second pulse (P 3 ) which can be seen in  FIG. 3 , mux  214  allows the counter  216  to count pulses from divider  216 , and during the logic low period T M  between first pulse (P 1 ) and the second pulse (P 3 ), mux  214  allows counter  216  to count pulses from the clock signal CLK. This allows the period detector  208  to determine the period (T) between the centers of consecutive pulses of the binary output of comparator  206  (P 1 /2+P 2 +P 1 /2). The period T can then be provided to controller  210 . 
     In the driving mode, controller  210  sets the mode signal MODE to logic low. As a result, corresponding to pulse of drive signal DRIVE, gate driver  202  alternately turns transistors Q 1  and Q 2  “on” and “off” Thus, a driving signal is supplied to transducer PZT. Additionally, because transistor Q 3  is “off,” comparator  206  does not receive a signal from node N 1 . 
     As a result of the operation in these two modes, controller  210  can adjust the signal applied to transducer PZT. Preferably, controller  210  compares the period of driving signal of the transducer PZT in the driving mode (i.e., the period of the drive signal DRIVE) with the measured period from period detector  208  in the measurement mode. Based on this comparison, controller  11  changes the output period (i.e., cycle number) of driving signal for the transducer PZT in the driving mode. Essentially, the driving mode and measurement mode are alternated, and the period of the drive signal DRIVE is adjusted based on the difference. 
     In one example, the difference between period of the driving signal of transducer PZT in the driving mode and the measured period from the period detector  208  in the measurement mode exceeds threshold E 1 . Following the comparison, controller  210  outputs one cycle of drive signal DRIVE in the driving mode. After output of one cycle of drive signal DRIVE, the operating mode is switched by controller  210  to the measurement mode. Thus, controller  210  reduces the output period of drive signal DRIVE and, more frequently, switches the operating mode between the driving mode and the measurement mode. Additionally, when this process is consecutively repeated for a predetermined number of rounds, controller outputs a signal indicating an abnormality in vibration of transducer PZT. 
     In another example, the difference between period of the driving signal of transducer PZT in the driving mode and the measured period from the period detector  208  in the measurement mode is below threshold E 1 . Following the comparison, controller  210  sets the output period of drive signal DRIVE to a predetermined number of cycles (e.g., 100 cycles). Thus, controller  210  prolongs the output period of drive signal DRIVE and reduces the frequency for generating the measurement mode. 
     In operation, controller  210  generates the drive signal DRIVE and the mode signal MODE based at least on part on the output from period detector  208 . Register  232  stores setting value of the period T of drive signal DRIVE. Register  232  holds data selected by mux  230  according to control signal from sequence controller  244 . Preferably, the sequence controller  244  selects (through mux  230 ) detected value from period detector  208 , lower threshold T MIN  of drive signal DRIVE stored in register  226 , and the sum of value held in register  232  and an increment value (α) obtained in adder  228 . This selection can be based at least on part on a comparison by comparator  222  between the standard range of detected values of period detector (stored in register  220 ) and the detected value from the period detector  208 . Additionally, this selection can be based at least in part on a comparison by comparator  224  between the detected value from period detector  208  and the value held in register  232 , outputting a signal indicating whether difference exceeds threshold E 1 . Frequency divider  234  divides clock signal CLK by the frequency dividing number corresponding to input setting value (value stored in register  232 ) and outputs the frequency divided signal as drive signal DRIVE. Edge detector  218  detects the rising edge and falling edge of the signal output from comparator  206 . Counter  240  counts the pulses of drive signal DRIVE, and when the counted value reaches an assigned by sequence controller  244 , output signal from counter  240  is changed from 0 to 1, while also resetting the count value to zero. Counter  236  counts the number of refresh rounds of value of register  232 , while also resetting it count value to zero when a reset signal is received from sequence controller  244 . Flip-flop  238  holds control signal output from sequence controller  244  in synchronization with the falling edge of drive signal DRIVE. Flip-flop  242  sets mode signal MODE to 1 when output signal of counter  240  changes from 0 to 1 and resets mode signal MODE to 0 when the signal from flip-flop  238  becomes 1. The sequence controller  244  controls period detector  208  based on detection signal of edge detector  218  in the measurement mode so that the interval between the i th  pulse (i represents an integer of 1 or larger) and the (i+1) th  pulse of signal binary signal from comparator  206  is detected. 
     An example operation for the driver  200  can be seen using the timing diagram of  FIG. 3 . For period detector  208 , sequence controller  244  controls counter  216  such that the pulses output from mux  214  in the period from the rising edge of the first pulse (P 1 ) to the falling edge of the second (P 3 ) pulse are calculated. Assuming i=2, sequence controller  244  resets the counted value of counter  216  to zero before moving from the driving mode to the measurement mode, and after shifting to the measurement mode, counter  216  counts between the third edge and the sixth edge of signal from comparator  206 . Because signal from comparator  206  is reset to the low level at the start of the measurement mode, the third edge corresponds to the rising edge of the second pulse, and the sixth edge corresponds to the falling edge of the third pulse. After the detection value (from counter  216 ) is obtained, sequence controller  244  controls mux  230 , register  232 , and counter  240  corresponding to signals from comparators  224  and  222 . When comparator  222  indicates that detected value from period detector  208  is out of standard range, sequence controller  244  starts a sequence for searching for an appropriate value for setting value by gradually increasing setting value from lower threshold T MIN . When a new search sequence is started, sequence controller  244  selects lower threshold T MIN  stored in register  226  and stores it in register  232 . Subsequently, sequence controller  244  selects the data output from adder  228  (which increment the value held in register  232  by value α) and stores it in register  232 . Additionally, sequence controller  244  sets the assigned value of counter  240  at loop number  1  in the search sequence. When detected value from period detector  208  falls within the standard range, sequence controller  244  uses the output signal from comparator  224  as a reference. When comparator  224  indicates that the difference between setting value and detected value exceeds threshold E 1 , sequence controller  244  selects detected value of period detector  208  and stores it in register  232 , while assigning a value to counter  240  at loop number  1 . On the other hand, when comparator  224  indicates that the difference between setting value and detected value is below threshold E 1 , sequence controller  244  selects detected value of period detector  208  and stores it in register  232 , while assigning a value to counter  240  at loop number  100 . 
     When the assigned value of counter  240  is set to  100 , sequence controller  244  resets the counted value of counter  236  to zero. When the assigned value of counter  240  is 1, the number of rounds of refresh of setting value of register  232 , that is, the number of rounds of consecutively repeating the driving mode and the measurement mode, is counted by counter  236 . When counted value of counter  236  reaches a predetermined value, signal S 13  indicating abnormality in vibration of transducer PZT is output under control of sequence controller  244 . When setting value of the period of drive signal DRIVE is established, that is, after completion of the search sequence, sequence controller  244  changes control signal (input into flip-flop  238 ) from 0 to 1. When this control signal becomes 1, at the falling edge of drive signal DRIVE, the output signal from flip-flop  238  becomes 1, which causes mode signal MODE to be reset to 0, and the operating mode is changed from the measurement mode to the driving mode. When the operating mode is switched to the driving mode, sequence controller  244  refreshes the frequency dividing number in frequency divider  234  to setting value Ts of register  232 , and resets the counted value of counter  240  to zero. 
     Now turning to  FIG. 4 , a flow chart of the operation of the driver  200  can be seen. In the first step ST 101 , sequence controller sets state flag to 1, sets count value for counter  236  to 0, sets mode signal MODE to 1, and sets the value in register  232  to the lower threshold T MIN . 
     After initialization in step ST 101 , the operating mode of the driver is switched to the driving mode for one cycle in step ST 102 . In step ST 102 , the frequency dividing operation of frequency divider  234  and the counting operation of counter  240  are started, and the assigned value of counter  240  is set to 1. When mode signal MODE becomes low or 0, gate driver  202  enters the active state, and transistors Q 1  and Q 2  are alternately turned ON corresponding to drive signal DRIVE. At output node N 1 , driving signal is generated corresponding to drive signal DRIVE. Here, transistor Q 3  is turned OFF, and comparator  206  is turned OFF. When 1-cycle (1-pulse) driving signal at node N 1  is output, count value of counter  240  becomes equal to the assigned value 1. As a result, mode signal MODE becomes 1 or high, and the operating mode is switched from the driving mode to the measurement mode. 
     After entering the measurement mode, detection occurs in step ST 103 . In measurement mode, transistors Q 1  and Q 2  are turned OFF, and node N 1  enters the high impedance state. Also, transistor Q 3  is turned ON, and comparator  206  is enabled. When supply of driving signal is stopped, under the electric energy received up to that point, transducer PZT freely vibrates. The pressure generated by the free vibration is applied to piezoelectric element  102  of transducer PZT, generating an electric signal having the same period as that of the free vibration. This electric signal is input via transistor Q 3  to comparator  206 , and it is transformed to binary signal. The period of signal at node N 3  corresponds to the period of the free vibration of transducer PZT. In period detector  208 , in order to avoid detecting a period with a discontinuous waveform just after transition to the measurement mode, for example, a predetermined number of pulses at the start is excluded from the object of detection. 
     When detection value is obtained by period detector  208 , controller  210  can then make several determinations. In step ST 104 , controller  210  judges whether detected value is contained in the predetermined standard range. If detected value from period detector  208  is out of standard range, a determination of the state (flag value) is made in step ST 108 . If flag value 1, the value held within register  232  is incremented by a value a in step ST 111 . If flag value is 0, a lower threshold T MIN  is stored in register  232  in step ST 109 , and the flag is subsequently set to 1 in step ST 110 . When setting value of register  232  is refreshed, count value of counter  236  is incremented by only 1 in step ST 113 , and a determination is made as to whether the count value of counter  236  has reached an upper limit or maximum value in step ST 114 . If count value of counter  236  has not reached the upper limit, then the process begins again in step ST 102 . Otherwise, an error is returned in step ST 115 . 
     Turning back to step ST 104 , when detected value is contained in standard range, a determination is made with respect to threshold E 1  in step  105 . Specifically, a determination as to whether the absolute different between the value held in register  232  and the detected value from period detector  208  exceeds threshold E 1  is made. If the difference exceeds threshold E 1 , detected value is selected as the new setting value to be stored in register  232  in step ST 112 . That is, detected value of period detector  208  is stored in register  232  selected by mux  230 . Otherwise, as this difference falls below threshold E 1 , flag F is reset to 0, and the search sequence is ended in step ST 106 . Additionally, count value of counter  236  (number of rounds of refresh of setting value) is reset to zero, and detected value is set as the new setting value (stored in register  232 ). The assigned value of counter  240  is changed from 1 to 100, and the operating mode is switched to the driving mode in step ST 107 . Here, in the driving mode, 100 cycles (100 pulses) of continuous driving signal on node N 1  are output. Then, after output of the 100 cycles, the operating mode is switched again to the measurement mode in step ST 103 , and the process is repeated. 
       FIG. 5  is a diagram is an example of a timing diagram for various portions of the driver. In particular, the mode signal MODE and the signals at nodes N 1 , N 2 , and N 3  are shown. Additionally,  FIG. 6  is a timing diagram illustrating several signals and nodes within controller  210 . In particular, signals MODE, DRIVE, and EN are shown along with the signals at nodes N 3 , N 4 , N 5 , and N 6 . 
     Additionally, the period of driving signal S 1  in the driving mode is set based on detected value of period detector  208  in the measurement mode just before the driving mode (step ST 112  shown in  FIG. 4 ). However, the present invention is not limited to this scheme. As another scheme for embodiment of the present invention, the period of the driving signal may be set based on the detected value of period detector  208  in a series of plural rounds of the measurement mode performed repeatedly until just before the driving mode. For example, one may adopt a scheme in which a digital filter (such as a filter that multiplies a predetermined weighting coefficient to a series of plural detected values and then adds them up) is set to receive as inputs the plural detected values in a series of plural measurement mode performed repeatedly until just before the driving mode, and its output is taken as the setting value of the period of the driving signal. 
     Additionally, when the driving mode, in which 1-cycle driving signal is output, is repeated continuously for a predetermined number of rounds (when the driving mode and the measurement mode are consecutively repeated for a predetermined number of rounds), signal at node N 7  indicating abnormality in vibration of transducer PZT is output. However, the present invention is not limited to this scheme. For example, as another scheme that may be adopted in the present invention, signal output from sequence controller at node N 7  is output when the driving mode that outputs a 1-cycle driving signal is executed repeatedly for a predetermined time (when the driving mode and the measurement mode are continuously repeated for a predetermined time). 
     Additionally, when the difference between setting value of driving signal at node N 1  in the driving mode and detected value of the period of free vibration of transducer PZT in the measurement mode is larger than threshold E 1 , 1-cycle driving signal S 1  is output in the next driving mode (in  FIG. 4 , steps ST 105 , ST 112 -ST 102 ). However, the cycle number (part number) can be selected as desired, and it may be 2 or more cycles (2 or more pulses). 
     Additionally, when the difference between setting value Ts of driving signal S 1  in the driving mode and detected value Td of the period of free vibration of transducer PZT in the measurement mode is smaller than threshold E 1 , 100 cycles of driving signal S 1  (100 pulses) in the next driving mode are output (steps ST 105 -ST 107  in  FIG. 4 ). However, this cycle number may be any larger number if the difference exceeds threshold E 1 . 
     Additionally, if the difference is smaller than a predetermined threshold, the period of the next driving mode is defined by the number of cycles of the driving signal. However, the present invention is not limited to this scheme. For example, as another scheme that may be adopted in the present invention, the period of the driving mode may also be defined by time. That is, after output of the driving mode for a predetermined time (a time longer than that if the difference exceeds threshold E 1 ), the operating mode is switched to the measurement mode. 
     Additionally, even if the difference is smaller than the predetermined threshold, the measurement mode is still executed on a regular basis. However, the present invention is not limited to this scheme. For example, as another scheme that may also be adopted in the present invention, if the difference is smaller than a predetermined threshold, the operating mode is kept in the driving mode. In this case, one may also adopt a scheme in which the control part executes the measurement mode under instruction from an upper-level device instead of executing the measurement mode on a regular basis. 
     Additionally, when the difference between setting value Ts and detected value Td is smaller than threshold E 1 , the operating mode is switched to the driving mode in which 100 cycles of driving signal (100 pulses) are continuously supplied. However, the present invention is not limited to this scheme. For example, one may also adopt a scheme in which, when the difference between setting value and detected value is continuously smaller than threshold E 1  for a predetermined number of rounds (e.g., for two or more rounds), the operating mode is switched to the driving mode, in which 100 cycles of driving signal (100 pulses) are continuously supplied. 
     Additionally, when detected value is outside the standard range, the period of driving signal in the next round of the driving mode is set at a predetermined lower threshold min (lower threshold of standard range), and then, during the period until the difference falls below threshold E 1 , when detected value is outside standard range, the period of driving signal in the next round of the driving mode is set at a value larger by increment a than the period of driving signal in the last round of the driving mode (steps ST 108 , ST 109 , ST 110 , ST 111 ). However, the present invention is not limited to this scheme. 
     For example, as another embodiment of the present invention, the following scheme may be adopted: when the difference between setting value and detected value is larger than threshold E 2  that is higher than threshold E 1  (that is, when setting value is significantly larger than the optimum value), the period of driving signal in the next round of the driving mode is set at a predetermined lower threshold T MIN , and then, during the period until the difference falls below threshold E 1 , if the difference is over the threshold E 2 , the period of driving signal S 1  in the next round of the driving mode is set to a value larger by increment a than the period of driving signal in the last round of the driving mode. 
     As another scheme that may also be adopted in the present invention, the lower threshold T MIN  is changed to the upper threshold T MIN  (upper threshold of standard range), and the increment α is changed to decrement β. That is, one may also adopt a scheme in which setting value of the period is obtained by subtracting decrement β from upper threshold T MAX . 
     In the example, the lower threshold T MIN  and upper threshold T MAX  are taken as the lower threshold and upper threshold of standard range. However, the present invention is not limited to this scheme. For example, lower threshold T MIN  and upper threshold T MAX  may be taken as the lower threshold and upper threshold of any range in standard range. 
     Additionally, setting value Ts of driving signal S 1  is increased stepwise by increment a from lower threshold T MIN  (step ST 111 ). However, the present invention is not limited to this scheme. For example, one may also adopt a scheme in which, after step ST 111  shown in  FIG. 4 , the following steps are arranged: a step in which whether setting value has reached predetermined upper threshold T MAX  is judged, and a step in which setting value is returned to the lower threshold T MIN  when setting value reaches upper threshold T MAX . In this way, repeated searching for the optimum value of setting value in the range from lower threshold T MAX  to upper threshold T MAX  is possible. 
     Similarly, one may also adopt a scheme in which, in the embodiment with stepwise decrease of setting value Ts by decrement β from upper threshold T MAX , the following steps are arranged: a step in which whether setting value has reached lower threshold T MIN  is judged, and a step in which when setting value reaches lower threshold T MIN , setting value is returned to upper threshold T MAX . Also in this case, repeated searching of the optimum value of setting value Ts in the range from lower threshold T MIN  to upper threshold T MAX  is possible. 
     In addition, when the driving mode in which the driving signal is supplied continuously cannot be switched ON even if searching is executed between lower threshold T MIN  and upper threshold T MAX , one may change lower threshold T MIN , upper threshold T MAX  and standard range so that standard range is wider. In this case, threshold E 1  may be increased. 
     Having thus described the present invention by reference to certain of its preferred embodiments, it is noted that the embodiments disclosed are illustrative rather than limiting in nature and that a wide range of variations, modifications, changes, and substitutions are contemplated in the foregoing disclosure and, in some instances, some features of the present invention may be employed without a corresponding use of the other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the scope of the invention.