Abstract:
A power generating device includes: a piezoelectric member that is formed from a piezoelectric material; one pair of electrodes that are installed to the piezoelectric member; a transformation unit that repeatedly transforms the piezoelectric member; an inductor that is installed between the one pair of electrodes and configures a resonance circuit together with a capacitive component of the piezoelectric member; a switch that is connected to the inductor in series; and a switch control unit that connects the switch when a transformation direction of the piezoelectric member is switched, and cuts off the switch when a time corresponding to a half period of a resonance period of the resonance circuit elapses.

Description:
BACKGROUND 
     1. Technical Field 
     The present invention relates to a power generating device that derives electric energy from an electric charge when a piezoelectric material such as a piezoelectric device is transformed by an external force. 
     2. Related Art 
     When a piezoelectric material such as lead zirconate titanate (PZT), crystal (SiO 2 ), or zinc oxide (ZnO) is transformed by an external force, electric polarization is caused inside the material, and positive and negative electric charge appears on the surface. Such a phenomenon is called a piezoelectric effect. A method of generating power is proposed in which, by using such a property of the piezoelectric material, a cantilever is vibrated so as to repeatedly weight the piezoelectric material, and an electric charge generated on the surface of the piezoelectric material is derived as electricity. 
     For example, by vibrating a metal cantilever which has a spindle arranged at its tip end and to which a thin plate of a piezoelectric material is attached, positive and negative electric charge alternately generated in the piezoelectric material in accordance with the vibration is taken out, thereby generating an AC current. A technique is proposed in which such an AC current is rectified by a diode, then stored in a capacitor, and taken out as electric power (JP-A-7-107752). In addition, a technique is proposed in which a DC current is acquired without incurring a voltage loss in a diode by closing the contact point only while positive charge is generated in the piezoelectric device (JP-A-2005-312269). By using such techniques, the power generating device can be miniaturized. Accordingly, for example, there are hopes for an application in which the power generating device is built into a small-sized electronic component instead of a battery cell, or the like. 
     However, according to the proposed general techniques, there is a problem that the acquired voltage is limited to the level of a voltage that is generated in accordance with electric polarization of the piezoelectric material. Accordingly, in most cases, a separate booster circuit is necessary, and there is a problem that it is difficult to decrease the size of the power generating device. 
     SUMMARY 
     An advantage of some aspects of the invention is that it provides a technique for generating a high voltage without increasing the size of a power generating device using the piezoelectric effect of a piezoelectric material. 
     An aspect of the invention is directed to a power generating device that derives a current from an electric charge generated by a piezoelectric member by transforming the piezoelectric member formed from a piezoelectric material by using an external force. The power generating device includes: a transformation unit that repeatedly transforms the piezoelectric member; one pair of electrodes that are installed to the piezoelectric member; an inductor that is installed between the one pair of electrodes and configures a resonance circuit together with a capacitive component of the piezoelectric member; a switch that is connected to the inductor in series; and a switch control unit that connects the switch when a transformation direction of the piezoelectric member is switched, and cuts off the switch when a time corresponding to a half period of a resonance period of the resonance circuit elapses. 
     In the above-described power generating device, since the piezoelectric member is repeatedly transformed. As a result, in the piezoelectric member, positive and negative electric charge is generated based on the piezoelectric effect. In addition, the amount of generation of the electric charge increases as the amount of transformation of the piezoelectric member increases. When the magnitude of the amount of transformation is the maximum (in other words, when the transformation direction is switched), the piezoelectric member is connected to the inductor. Since the piezoelectric member can be regarded as a capacitor in the sense of an electric circuit, by connecting the piezoelectric member to the inductor, a resonance circuit is formed. Then, the electric charges generated in the piezoelectric member flow into the inductor. Since the piezoelectric member and the inductor configures a resonance circuit, the current flowing into the inductor overshoots and flows into the piezoelectric member from the opposite terminal. This period (that is, a period until the electric charge started to flow from one terminal of the piezoelectric member flows into the piezoelectric material again from the other terminal disposed on the opposite side through the inductor) is a half of the resonance period of the resonance circuit that is formed by the piezoelectric member and the inductor. Accordingly, when the piezoelectric member and the inductor are connected at the timing at which the transformation direction of the piezoelectric member is switched, and thereafter, the inductor is cut off from the piezoelectric member at the timing at which a time corresponding to a half of the resonance period elapses, the arrangement of positive and negative charge generated inside the piezoelectric member before the connection between the piezoelectric member and the inductor can be reversed. From the state, this time, when the piezoelectric member is transformed in the opposite direction, the electric charge generated based on the piezoelectric effect can be accumulated inside the piezoelectric member. As a result, the electric charge generated by repeatedly transforming the piezoelectric member can be effectively accumulated inside the piezoelectric member. In addition, as the electric charge is accumulated inside the piezoelectric member, the voltage between the terminals increases. Accordingly, a voltage that is higher than a voltage generated due to electric polarization of the piezoelectric material can be generated without separately arranging a booster circuit. Therefore, a small-size power generating device having excellent efficiency can be acquired. 
     In the above-described power generating device, a rectifier circuit that rectifies an AC current generated by the piezoelectric member may be connected to the one pair of electrodes disposed in the piezoelectric member. 
     Since the piezoelectric member is repeatedly transformed, an AC current is generated from one pair of electrodes disposed in the piezoelectric member. Accordingly, by connecting the rectifier circuit to one pair of electrodes, the AC current can be converted into a DC current so as to drive various electric loads. 
     In the above-described power generating device, it may be configured such that the presence of a current flowing through the rectifier circuit from the piezoelectric member is detected, and the switch is connected when no current flows. 
     When the transformation direction of the piezoelectric member is switched, there is no current flowing through the rectifier circuit from the piezoelectric member. Accordingly, by connecting the switch so as to connect the piezoelectric member and the inductor to each other when no current flows to the rectifier circuit from the piezoelectric member, the switch can be connected at appropriate timing with high accuracy. As a result, since the piezoelectric member and the inductor can be connected when the electric charge generated by the piezoelectric member becomes the maximum, the electric charge generated in the piezoelectric material can be efficiently taken out, whereby power can be efficiently generated. 
     In a case where the presence of the current flowing through the rectifier circuit from the piezoelectric member is detected, the detection may be performed by using a photocoupler. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
         FIGS. 1A and 1B  are explanatory diagrams showing the structure of a power generating device according to this embodiment. 
         FIGS. 2A to 2D  are explanatory diagrams showing the operation of the power generating device according to this embodiment. 
         FIGS. 3A to 3F  are explanatory diagrams conceptually illustrating a first half part of the operating principle of the power generating device according to this embodiment. 
         FIGS. 4A to 4F  are explanatory diagrams conceptually illustrating a second half part of the operating principle of the power generating device according to this embodiment. 
         FIGS. 5A to 5E  are explanatory diagrams illustrating the reason that a switch can be controlled at appropriate timing by detecting a current flowing from a piezoelectric member to a full-wave rectifier circuit. 
         FIG. 6  is an explanatory diagram illustrating the circuit configuration of a power generating device according to a modified example in which a photocoupler is built in a part of the full-wave rectifier circuit. 
         FIG. 7  is an explanatory diagram showing another form of the modified example in which a photocoupler is built between the piezoelectric member and the full-wave rectifier circuit. 
     
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     Hereinafter, in order to clarify the content of the invention, embodiments will be described in the following order.
     A. Structure of Power Generating Device   B. Operation of Power Generating Device   C. Operating Principle of Power Generating Device   D. Switch Switching Timing   E. Modified Examples   

     A. Structure of Power Generating Device 
       FIGS. 1A and 1B  are explanatory diagrams illustrating the structure of a power generating device  100  according to this embodiment.  FIG. 1A  shows the mechanical structure of the power generating device  100 , and  FIG. 1B  shows the electrical configuration thereof. As the mechanical structure of the power generating device  100  according to this embodiment, a structure is formed in which a crossbeam  104  having a spindle  106  disposed at the tip end is fixed to a support end  102  on the base end side. On the surface of the crossbeam  104 , a piezoelectric member  108  that is formed from a piezoelectric material such as lead zirconate titanium (PZT) is disposed. In addition, on the surface of the piezoelectric member  108 , on the front and rear sides, an upper electrode  109   a  and a lower electrode  109   b  that are formed by a metal foil are disposed, respectively. In the example shown in  FIG. 1A , although the piezoelectric member  108  is disposed on the upper face side of the crossbeam  104 , the piezoelectric member  108  may be disposed on the lower face side of the crossbeam  104 , or the piezoelectric member  108  may be disposed on both the upper face side and the lower face side of the crossbeam  104 . In addition, since the piezoelectric member  108  is transformed in accordance with the transformation of the crossbeam  104 , the crossbeam  104  corresponds to a “transformation unit” according to an embodiment of the invention. 
     In the crossbeam  104 , the base end side is fixed to the support end  102 , and the spindle  106  is disposed on the tip end side. Accordingly, by adding vibration thereto or the like, the tip end of the crossbeam  104  greatly vibrates as denoted by a white arrow in the figure. As a result, a compressive force and a tensile force are alternately applied to the piezoelectric member  108  attached to the surface of the crossbeam  104 . Then, the piezoelectric member  108  generates positive and negative electric charge based on the piezoelectric effect, and the electric charge appears in the upper electrode  109   a  and the lower electrode  109   b.    
       FIG. 1B  shows the circuit diagram of the power generating device  100  according to this embodiment. The piezoelectric member  108  can be electrically represented as a current source and a capacitor C 0  that stores electric charge. An inductor L is connected to the piezoelectric member  108  in parallel and forms an electric resonance circuit together with a capacitive component of the piezoelectric member  108 . A switch SW used for turning On/Off the resonance circuit is connected to the inductor L in series. The turning On/Off of the switch SW is controlled by a control circuit  112 . In addition, the upper electrode  109   a  and the lower electrode  109   b  disposed in the piezoelectric member  108  are connected to a full-wave rectifier circuit  120  that is configured by four diodes D 1  to D 4  through a current detecting circuit  110  to be described later. A capacitor (an output capacitor C 1 ) that stores a current after rectification is connected to the full-wave rectifier circuit  120 . 
     B. Operation of Power Generating Device 
       FIGS. 2A to 2D  are explanatory diagrams illustrating the operation of the power generating device  100  according to this embodiment.  FIG. 2A  illustrates the appearance of change in the displacement u of the tip end of the crossbeam  104  in accordance with the vibration of the crossbeam  104 . A positive displacement u represents a state in which the crossbeam  104  is in an upwardly warped shape (a state in which the top face side of the crossbeam  104  is indented), and a negative displacement (−u) represents that the crossbeam  104  is in a downwardly warped state (a state in which the bottom face of the crossbeam  104  is indented).  FIG. 2B  illustrates the appearance of a current generated by the piezoelectric member  108  in accordance with the transformation of the crossbeam  104  and a resultant electromotive force generated inside the piezoelectric member  108 . In  FIG. 2B , the appearance of the generation of electric charge in the piezoelectric member  108  is represented as the amount of electric charge generated per unit time (that is, a current Ipzt), and the electromotive force generated in the piezoelectric member  108  is represented by an electric potential difference Vpzt generated between the upper electrode  109   a  and the lower electrode  109   b.    
     As shown in  FIGS. 2A and 2B , while the displacement of the crossbeam  104  increases, the piezoelectric member  108  generates a current (in other words, a positive value of the current Ipzt) in the positive direction, and the electric potential difference Vpzt between the upper electrode  109   a  and the lower electrode  109   b  increases in the positive direction accordingly. When the electric potential difference Vpzt in the positive direction is larger than the sum of the voltage VC 1  of the output capacitor C 1  and twice the forward drop voltage Vf of the diode configuring the full-wave rectifier circuit  120 , that is, VC 1 +2Vf, electric charge generated thereafter can be derived as a DC current and stored in the output capacitor C 1 . On the other hand, while the displacement of the crossbeam  104  decreases, the piezoelectric member  108  generates a current (in other words, a negative value of the current Ipzt) in the negative direction, and the electric potential difference Vpzt between the upper electrode  109   a  and the lower electrode  109   b  increases in the negative direction accordingly. When the electric potential difference Vpzt in the negative direction is larger than the sum of the voltage VC 1  and twice the forward drop voltage 2Vf of the full-wave rectifier circuit  120 , generated electric charge can be derived as a DC current and stored in the output capacitor C 1 . In other words, even in a state in which the switch SW shown in  FIGS. 1A and 1B  is turned off, for a portion denoted by applying diagonal lines thereto in  FIG. 2B , electric charge can be stored in the output capacitor C 1 . 
     In the power generating device  100  according to this embodiment, the switch SW is turned on at the timing as shown in  FIG. 2C . Then, as shown in  FIG. 2D , a phenomenon occurs in which the voltage waveform between the terminals of piezoelectric member  108  is shifted at the moment the switch SW is turned on. In other words, for example, in a period B denoted by “B” in  FIG. 2D , a voltage waveform denoted by a thick broken line that is acquired by shifting a voltage waveform Vpzt, which is denoted by a thin broken line, corresponding to the electromotive force of the piezoelectric member  108  in the negative direction appears between the terminals of the piezoelectric member  108 . The reason for the occurrence of such a phenomenon will be described later. In addition, in a period C denoted by “C” in the  FIG. 2D , a voltage waveform, which is denoted by a thick broken line, acquired by shifting the voltage waveform Vpzt corresponding to the electromotive force of the piezoelectric member  108  in the positive direction appears. In the following periods D, E, and F, and the like, similarly, a voltage waveform denoted by a thick broken line that is acquired by shifting the voltage waveform Vpzt corresponding to the electromotive force of the piezoelectric member  108  in the positive direction or the negative direction appears. For a portion in which the shifted voltage waveform exceeds the sum of VC 1  and 2Vf (a portion denoted by applying diagonal lines thereto in  FIG. 2D ), the electric charge generated by the piezoelectric member  108  can be stored in the output capacitor C 1 . As the electric charge flows from the piezoelectric member  108  to the output capacitor C 1 , consequently, the voltage between the terminals of the piezoelectric member  108  is clipped to the voltage of the sum of VC 1  and 2Vf. As a result, the voltage waveform between the upper electrode  109   a  and the lower electrode  109   b  is a waveform denoted by a thick solid line in  FIG. 2D . 
     As is apparent by comparing a case shown in  FIG. 2B  where the switch SW is maintained to be turned off and a case shown in  FIG. 2D  where the switch SW is turned on at the timing the transformation direction of the crossbeam  104  is switched, according to the power generating device  100  of this embodiment, by turning the switch SW on at appropriate timing, electric charge can be efficiently stored in the output capacitor C 1 . 
     In the output capacitor C 1 , electric charge can be stored, and as the inter-terminal voltage of the output capacitor C 1  increases, the amount of shift of the voltage waveform increases in accordance therewith. For example, when period B (a state in which electric charge is not stored in the output capacitor C 1 ) shown in  FIG. 2D  and period H (a state in which a small amount of electric charge is stored in the output capacitor C 1 ) shown in  FIG. 2D  are compared with each other, the amount of shift of the voltage waveform in period H is larger than that in period B. Similarly, when period C and period I shown in  FIG. 2D  are compared with each other, the amount of shift of the voltage waveform in period I in which the amount of electric charge stored in the output capacitor C 1  increases is larger than that in period C. The reason for the occurrence of such a phenomenon will be described later. As a result, according to the power generating device  100  of this embodiment, by transforming the piezoelectric member  108 , a voltage that is equal to or higher than the voltage Vpzt generated between the upper electrode  109   a  and the lower electrode  109   b  can be stored in the output capacitor C 1 . Consequently, a special booster circuit does not need to be arranged, and accordingly, a high-efficiency small-size power generating device can be acquired. 
     C. Operating Principle of Power Generating Device 
       FIGS. 3A to 3F  are explanatory diagrams conceptually illustrating a first half part of the principle of the operation of the power generating device  100  according to this embodiment.  FIGS. 4A to 4F  are explanatory diagrams conceptually illustrating a second half part of the principle of the operation of the power generating device  100  according to this embodiment. In  FIGS. 3A to 3F , the movement of electric charge inside C 0  at the time when the switch SW is turned on in accordance with the transformation of the piezoelectric member  108  is conceptually illustrated.  FIG. 3A  shows a state in which the piezoelectric member  108  (more precisely, the crossbeam  104 ) is upwardly transformed (the upper face side is indented). When the piezoelectric member  108  is upwardly transformed, a current flows in the positive direction from a current source, and accordingly, electric charge is stored in C 0 , thereby generating a voltage Vgen in the positive direction. The voltage value increases as the transformation of the piezoelectric member  108  increases. Then, the switch SW is turned on at the timing transformation of the piezoelectric member  108  is the maximum (timing the amount of electric charge is the maximum (see  FIG. 3B )). 
       FIG. 3C  shows a state immediately after the switch SW is turned on. In C 0 , electric charge is stored, and accordingly, the electric charge tends to flow through the inductor L. When a current flows through the inductor L, a magnetic flux is generated (a magnetic flux increases), and the inductor L has a property (self-induction action) of generating a counter electromotive force in a direction hindering a change in the magnetic flux passing through it. At the moment the switch SW is turned on, the magnetic flux tends to increase due to flow of electric charge, and a counter electromotive force is generated in a direction (in other words, a direction hindering the flow of electric charge) hindering an increase in the magnetic flux. The magnitude of the counter electromotive force is in proportion to the speed of change in the magnetic flux (the amount of change per unit time). In  FIG. 3C , the counter electromotive force generated in the inductor L as above is represented by a hatched arrow. Since such an electromotive force is generated, even when the switch SW is turned on, only little electric charge of the piezoelectric member  108  starts to flow. In other words, the current flowing through the inductor L increases little. 
     Thereafter, when the current flowing through the inductor L reaches its peak, the speed of change in the magnetic flux becomes “0”, and as shown in  FIG. 3D , the counter electromotive force becomes “0”. Then, in turn, the current starts to decrease. Then, since the magnetic flux passing through the inductor L decreases, an electromotive force is generated in the inductor L in a direction (a direction allowing the current to flow) hindering the decrease in the magnetic flux (see  FIG. 3E ). As a result, while electric charge is drawn out from C 0 , a current continuously flows through the inductor L. In a case where there is no loss in the middle of the movement of electric charge, all the electric charge generated in accordance with the transformation of the piezoelectric member  108  moves, and a state (in other words, a state is formed in which positive electric charge is distributed on the lower face side of the piezoelectric member  108  and negative electric charge is distributed on the upper face side thereof) is formed in which the positive charge and the negative charge are displaced.  FIG. 3F  shows a state in which all the positive and negative charge generated in accordance with the transformation of the piezoelectric member  108  moves. 
     In this state, when the switch SW is maintained as turned on, this time, a phenomenon opposite to the description presented above occurs. In other words, positive charge disposed on the lower face side of the piezoelectric member  108  tends to flow through the inductor L, and at this time, a counter electromotive force is generated in the inductor L in a direction hindering the flow of the electric charge. Thereafter, after the current flowing through the inductor L reaches its peak and then decreases, at this time, an electromotive force in a direction (a direction allowing the current to continuously flow) hindering the decrease in the current is generated in the inductor L. As a result, a state is formed in which all the positive charge disposed on the lower face side of the piezoelectric member  108  moves to the upper face side (the state shown in  FIG. 3B ). The positive charge which has been returned to the upper face side of the piezoelectric member  108  moves to the lower face side again as described with reference to  FIGS. 3B to 3F . 
     As above, when the switch SW is turned on in a state in which electric charge is stored in C 0 , and the state is maintained, a type of resonance phenomenon occurs in which the direction of the current alternately inverts between the piezoelectric member  108  and the inductor L. The period of this resonance phenomenon is the period T of a so-called LC resonance circuit, and thus, when the magnitude (capacitance) of the capacitive component C 0  included in the piezoelectric member  108  is C, and the magnitude (inductance) of the inductive element of the inductor L is L, the period T is given as T=2π(LC) 0.5 . Accordingly, a time from immediately after the switch SW is turned on (the state as shown in  FIG. 3B ) to a time the state as shown in  FIG. 3F  is formed is T/2. 
     Thus, at a time point when a time T/2 elapses after the switch SW is turned on, as shown in  FIG. 4A , the switch SW is turned off. Then, from this state, the piezoelectric member  108  (more precisely the crossbeam  104 ) is downwardly transformed in turn (the lower side is indented). In  FIG. 3A  described above, although the piezoelectric member  108  is upwardly transformed, in  FIG. 4A , the piezoelectric member is downwardly transformed. Accordingly, a current in the negative direction flows from the current source, and electric charge is accumulated in C 0  so as to increase Vgen in the negative direction. As described above with reference to  FIGS. 3A to 3F , in a stage before the piezoelectric member  108  (more precisely, the crossbeam  104 ) is downwardly transformed, positive charge is distributed on the lower face side of the piezoelectric member  108 , and negative charge is distributed on the upper face side, and accordingly, in addition to the electric charge, new positive charge is accumulated on the lower face side, and new negative charge is accumulated on the upper face side. In  FIG. 4B , a state is shown in which new electric charge is accumulated in the piezoelectric member  108  by transforming the piezoelectric member  108  (more precisely, the crossbeam  104 ) in the state in which the switch SW is turned off. 
     When the switch SW is turned on from this state, the positive charge accumulated on the lower face side of the piezoelectric member  108  tends to flow through the inductor L. At this time, since a counter electromotive force is generated in the inductor L (see  FIG. 4C ), the current start to flow a little, then reaches its peak, and decreases thereafter. Then, an electromotive force is generated in a direction hindering the decrease in the current (the direction allowing the current to continuously flow) in the inductor L (see  FIG. 4E ), the current continuously flows in accordance with the electromotive force, and finally, a state is formed in which all the positive charge distributed on the lower face side of the piezoelectric member  108  moves to the upper face side, and all the negative charge distributed on the upper face side moves to the lower face side (see  FIG. 4F ). In addition, a time required for moving all the positive charge disposed on the lower face side to the upper face side and moving all the negative charge disposed on the upper face side to the lower face side is a time T/2 corresponding to a half period of the LC resonance circuit. Thus, when the time T/2 elapses after the switch SW is turned on, the switch SW is turned off, and, at this time, in a case where the piezoelectric member  108  (more precisely, the crossbeam  104 ) is upwardly transformed (the upper face side is indented), the positive and negative electric charge can be accumulated further inside the piezoelectric member  108 . 
     As described above, according to the power generating device  100  of this embodiment, after electric charge is generated by transforming the piezoelectric member  108 , the piezoelectric member  108  is connected to the inductor L, and a resonance circuit is formed during a half period of the resonance period, whereby the distribution of the positive and negative electric charge is inverted inside the piezoelectric member  108 . Thereafter, the piezoelectric member  108  is transformed in the opposite direction so as to generate new electric charge. Since the distribution of the positive and negative electric charge is inverted inside the piezoelectric member  108 , the electric charge that is newly developed is accumulated in the piezoelectric member  108 . Thereafter, again, the piezoelectric member  108  is connected to the inductor L only for half the resonance period, the distribution of the positive and negative charge is inverted inside the piezoelectric member  108 , and then the piezoelectric member  108  is transformed in the opposite direction. By repeating such an operation, each time the piezoelectric member  108  is repeatedly transformed, the electric charge accumulated in the piezoelectric member  108  can be increased. 
     As described with reference to  FIGS. 2A to 2D , in the power generating device  100  of this embodiment, a special phenomenon occurs in which the voltage waveform between terminals of the piezoelectric member  108  is shifted each time the switch SW is turned on, and such a phenomenon is occurs through the above-described mechanism. In other words, for example, in period A shown in  FIG. 2D , although a voltage is generated between the upper electrode  109   a  and the lower electrode  109   b  in accordance with the transformation of the piezoelectric member  108  (more precisely, the crossbeam  104 ), the upper electrode  109   a  and the lower electrode  109   b  are connected to the full-wave rectifier circuit  120 , and accordingly, the electric charge exceeding a voltage corresponding to the sum of VC 1  and 2Vf flow into the output capacitor C 1  that is connected to the full-wave rectifier circuit  120 . As a result, when the switch SW is turned on at a time point at which the transformation of the crossbeam  104  becomes the maximum, the positive and negative charge remaining inside the piezoelectric member  108  at that time moves through the inductor L, whereby the arrangement of the positive and negative charge inside the piezoelectric member  108  is interchanged. As is apparent from the mechanism described with reference to  FIGS. 3A to 4F , the period in which the switch SW is turned on is a time corresponding to a half of the resonance period of the resonance circuit that is configured by the capacitive component of the piezoelectric member  108  and the inductor L. 
     When the crossbeam  104  is transformed in the opposite direction from the state in which the arrangement of the positive and negative electric charge is interchanged, a voltage waveform appears between the upper electrode  109   a  and the lower electrode  109   b  of the piezoelectric member  108  based on the piezoelectric effect. In other words, from the state in which the polarities of the upper electrode  109   a  and the lower electrode  109   b  of the piezoelectric member  108  are interchanged, a voltage change occurs due to the transformation of the piezoelectric member  108 . As a result, in period B shown in  FIG. 2D , a voltage waveform acquired by shifting the voltage waveform generated in the piezoelectric member  108  in accordance with the transformation of the crossbeam  104  appears. Above all, as described above, the electric charge exceeding the voltage corresponding to the sum of VC 1  and 2Vf flows into the output capacitor C 1 , and accordingly, the voltage between the upper electrode  109   a  and the lower electrode  109   b  of the piezoelectric member  108  is clipped to the voltage of the sum of VC 1  and 2Vf. Thereafter, when the switch SW is turned on for a time corresponding to a half of the resonance period, the arrangement of the positive and negative charge remaining in the piezoelectric member  108  is interchanged. Then, as the crossbeam  104  is transformed from that state, a voltage waveform based on the piezoelectric effect appears in the piezoelectric member  108 . Accordingly, also in period C shown in  FIG. 2D , a voltage waveform acquired by shifting the voltage waveform generated in accordance with the transformation of the crossbeam  104  appears. 
     In addition, as described above with reference to  FIGS. 2A to 2D , in the power generating device  100  of this embodiment, while the transformation of the crossbeam  104  repeats to be transformed, a phenomenon occurs in which the amount of the shift of the voltage waveform gradually increases. Accordingly, a remarkable advantage that a voltage higher than the electric potential difference generated between the upper electrode  109   a  and the lower electrode  109   b  can be stored in the output capacitor C 1  based on the piezoelectric effect of the piezoelectric member  108  can be acquired. Such a phenomenon occurs through the following mechanism. 
     First, as illustrated in period A or B shown in  FIG. 2D , in a case where the output capacitor C 1  is not charged, when the voltage generated between the terminals of the piezoelectric member  108  exceeds 2Vf of the full-wave rectifier circuit  120 , electric charge flows into the output capacitor C 1  from the piezoelectric member  108 , and the voltage appearing between the terminals of the piezoelectric member  108  is clipped to 2Vf. However, the voltage between the terminals of the output capacitor C 1  increases in accordance with the accumulation of electric charge in the output capacitor C 1  as above. Thereafter, the electric charge does not flow into from the piezoelectric member  108  until the inter-terminal voltage of the output capacitor C 1  becomes a voltage higher than the sum of VC 1  and 2Vf. Accordingly, the value at which the inter-terminal voltage of the piezoelectric member  108  is clipped gradually increases in accordance with the accumulation of electric charge in the output capacitor C 1 . 
     In addition, as described above with reference with  FIGS. 3A to 4F , so long as electric charge is not allowed to flow out of the piezoelectric member  108 , each time the piezoelectric member  108  (more precisely, the crossbeam  104 ) is transformed, the electric charge disposed inside the piezoelectric member  108  increases. Then, the inter-terminal voltage of the piezoelectric member  108  increases in accordance with the increase in the electric charge. Accordingly, the voltage of the inter-terminal voltage of the piezoelectric member  108  can be high in a case where the loss or the like is not considered, which occurs when the electric charge flows though the inductor L or the switch SW. Therefore, according to the power generating device  100  of this embodiment, power can be generated in a state in which the voltage is naturally raised up to a voltage level necessary for driving an electric load without arranging a special booster circuit. 
     D. Switch Switching Timing 
     As described above, in the power generating device  100  of the this embodiment, repetitive transformation is applied to the piezoelectric member  108  (more precisely, the crossbeam  104 ), at the moment at which the transformation direction is switched, the piezoelectric member  108  is connected to the inductor L only for a time corresponding to a half of the resonance period, and accordingly, the efficiency is improved. In addition, since the booster circuit is not necessary, a superior advantage that the size of the device can be decreased in an easy manner can be acquired. Above all, it is not so easy to turn the switch SW on at the moment at which the transformation direction of the crossbeam  104  is switched. For example, the moment at which the transformation direction of the crossbeam  104  is switched is a moment at which the magnitude of the displacement of the crossbeam  104  is the maximum, and accordingly, the switch may be configured so as to be tuned on at the moment at which the crossbeam  104  is at the maximum displacement by using a mechanical contact point. However, in a case where the adjustment of the contact point is mismatched, the efficiency greatly decreases. Thus, in the power generating device  100  of the this embodiment, as shown in  FIG. 1B , the switch SW is controlled by detecting a current flowing through the full-wave rectifier circuit  120  from the piezoelectric member  108  by using the current detecting circuit  110 . 
       FIGS. 5A to 5E  are explanatory diagrams illustrating a reason that the switch SW can be controlled at appropriate timing by detecting the current flowing through the full-wave rectifier circuit  120  from the piezoelectric member  108 .  FIG. 5A  shows the displacement of the crossbeam  104 .  FIG. 5B  shows the appearance of a change in the amount of electric charge (that is, the current Ipzt) generated by the piezoelectric member  108  per unit time in accordance with the vibration of the crossbeam  104  and the appearance of a change in the electromotive force Vpzt generated in accordance with the current Ipzt. 
     As shown in the figure, as the displacement of the crossbeam  104  increases, the electromotive force Vpzt increases as well. When the electromotive force Vpzt is higher than the sum of the voltage VC 1  of the output capacitor C 1  and twice the forward drop voltage Vf of the diode configuring the full-wave rectifier circuit  120 , that is, VC 1 +2Vf, the generated electric charge flows through the full-wave rectifier circuit  120 . 
     In addition, at a moment at which the magnitude of the displacement of the crossbeam  104  becomes the maximum (in other words, a moment at which the transformation direction of the crossbeam  104  is switched), the direction of the current Ipzt generated by the piezoelectric member  108  is reversed. For example, in a state in which the piezoelectric member  108  generates a positive electromotive force, when the magnitude of the displacement of the crossbeam  104  becomes the maximum, the current Ipzt flowing in the positive direction is inverted in the negative direction. Accordingly, the electromotive force of the piezoelectric member  108  decreases, and the voltage thereof is lower than the sum of VC 1  and 2Vf, whereby the current that has flowed through the full-wave rectifier circuit  120  becomes unable to flow. Similarly, in a state in which a negative electromotive force is generated by the piezoelectric member  108 , as the current Ipzt flowing in the positive direction is generated, the current that has flowed through the full-wave rectifier circuit  120  becomes unable to flow. Accordingly, the moment at which the transformation direction of the crossbeam  104  is switched (the moment at which the magnitude of the displacement of the crossbeam  104  is the maximum) coincides with the moment at which a current becomes unable to flow through the full-wave rectifier circuit  120  from the piezoelectric member  108 . Therefore, as shown in  FIG. 1B , in a case where the current flowing through the full-wave rectifier circuit  120  is detected by using the current detecting circuit  110 , when no current is detected, the switch SW may be turned on only for a predetermined time (a half of the resonance period) from that moment. 
       FIG. 5C  illustrates the appearance in which the switch SW is turned on for a half of the resonance period by detecting that a current has not flowed through the full-wave rectifier circuit  120 .  FIG. 5D  shows a current flowing through the inductor L and a current (the current flowing through the full-wave rectifier circuit  120 ) detected by the current detecting circuit  110 , as a result of the operation of the switch. Here, between the piezoelectric member  108  and the inductor L, a current acquired by summing the two currents flows. 
     As shown in  FIG. 5D , when the switch SW is turned on, electric charge disposed inside the piezoelectric member  108  flows through the inductor L, and the arrangement of positive and negative charge inside the piezoelectric member  108  is switched. Then, as the piezoelectric member  108  is transformed in the state in which the arrangement of the electric charge is switched, between the terminals of the piezoelectric member  108 , a voltage waveform as shown in  FIG. 5E  is generated. Since this voltage waveform is the same as the voltage waveform described with reference to  FIG. 2D , the description thereof will be omitted here. As a result, as shown in  FIG. 5D , the current Ipzt generated by the piezoelectric member  108  can be efficiently stored in the output capacitor C 1 . 
     Finally, in this embodiment, the configurations of the current detecting circuit  110  and the control circuit  112  will be briefly described. Inside the current detecting circuit  110 , a general current detecting sensor such as a hall element is mounted. The output of the current detecting sensor is amplified and is converted into an absolute value. Then, by comparing the acquired absolute value with a predetermined threshold value (a positive value slightly greater than zero), a moment at which the absolute value is smaller than the threshold value may be detected. In addition, the switch SW may be controlled by generating a pulse having a time width corresponding to a half of the resonance period with the detected moment used as a trigger and inputting the pulse to the control circuit  112 . In order to generate a pulse having a time width corresponding to the half of the resonance period, a delay circuit may be used, or a time corresponding to the half of the resonance period may be measured by using a timer. In this embodiment, the current detecting circuit  110  and the full-wave rectifier circuit  120  correspond to a “switch control unit” according to an embodiment of the invention. 
     E. Modified Example 
     In the description presented above, the current detecting sensor such as a hall element is mounted inside the current detecting circuit  110 , and, by processing the output thereof, the timing at which no current has flowed is detected. However, some of the diodes D 1  to D 4  that configure the full-wave rectifier circuit  120  may be replaced by a photocoupler. In such a case, the presence of the current is directly detected by the photocoupler. 
       FIG. 6  is an explanatory diagram illustrating the circuit configuration of a power generating device  100  according to a modified example in which some of the diodes D 1  to D 4  configuring the full-wave rectifier circuit  120  are replaced by photocouplers. In the example shown in the figure, although the diodes D 2  and D 4  are replaced by the photocouplers, the invention is not limited thereto. Thus, the diodes D 1  and D 3  may be replaced by photocouplers, the diodes D 1  and D 4  may be replaced by photocouplers, or the diodes D 2  and D 3  may be replaced by photocouplers. In such a case, while a current flows through the full-wave rectifier circuit  120 , one of the photocouplers can detect that. Accordingly, the current detecting circuit  110  can instantly detect that no current has flowed through the full-wave rectifier circuit  120 . As a result, as described above, without installing a current detecting sensor such as a hall element or performing a complicated process such as amplifying of the output of a sensor and acquiring of the absolute value thereof or comparing of the acquired absolute value with a threshold value, the switch SW can be turned on at appropriate timing. 
     Alternatively, as shown in  FIG. 7 , it may be configured such that photocouplers are inserted between the upper electrode  109   a  and the lower electrode  109   b  and the full-wave rectifier circuit  120 , and the presence of the current flowing through the full-wave rectifier circuit  120  is detected. 
     As above, although this embodiment and modified examples have been described, the invention is not limited thereto and can be performed in various forms within the scope not departing from the concept thereof. 
     For example, in the above-described embodiment, the piezoelectric member  108  has been described to be installed to the crossbeam  104  having a cantilever structure. However, a member to which the piezoelectric member  108  is installed may be any member, as long as it can be repeatedly transformed easily in accordance with vibration or the like. For example, the piezoelectric member  108  may be installed to the surface of a thin film, or the piezoelectric member  108  may be installed to the side face of a helical spring. 
     This application claims priority to Japanese Patent Application No. 2010-254580, filed on Nov. 15, 2010, the entirety of which is hereby incorporated by reference.