Abstract:
A disclosed power generating device includes: a first magnetostrictive bar; a second magnetostrictive bar extending alongside the first magnetostrictive bar; a joint member coupling the first magnetostrictive bar and the second magnetostrictive bars; and a coil group including a first coil wound around the first magnetostrictive bar and a second coil wound around the second magnetostrictive bar, wherein the first coil and the second coil are connected in series.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation application of International Application PCT/JP2013/072025 filed on Aug. 16, 2013 and designated the U.S., the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The embodiments discussed herein are related to a power generating device and a sensor system. 
       BACKGROUND 
       [0003]    There have been studied a sensor system which includes sensors installed on monitoring targets such as bridges and tunnels so as to monitor these monitoring targets, and which monitors the abnormality of the monitoring targets based on signals wirelessly transmitted from the sensors. 
         [0004]    Such a sensor system requires a power supply to drive each sensor. However, when a battery is used as the power supply, there arises such a problem that the sensor cannot be driven after the battery&#39;s life ends. Moreover, disposal of such dead batteries leads to environmental damage. 
         [0005]    Accordingly, energy harvesting techniques are attracting attentions as the technique to provide power supplies to the sensors without using batteries. The energy harvesting techniques generate electric power from ambient energy such as heat, vibration, and radio waves. The energy harvesting techniques have a merit that it can generate electric power so long as the ambient energies exist. 
         [0006]    As a power generating device using such an energy harvesting technique, a power generating device using a magnetostrictive material has been proposed, for example. This power generating device induces electromotive force in a coil wound around a bar made of a magnetostrictive material by using a phenomenon in which the magnetic flux penetrating the bar changes when stress applied to the bar changes. 
         [0007]    When installed on a bridge or the like, the power generating device can generate electric power by using vibration of the bridge. 
         [0008]    However, the above-described power generating device has room for improvement in terms of efficiency of extracting generated electric power. 
         [0009]    Note that techniques related to the present application are disclosed in International Publication Pamphlet No. WO2011/158473, and Japanese Laid-open Patent Publication No. 2008-72862. 
       SUMMARY 
       [0010]    According to one aspect discussed herein, there is provided a power generating device, including: a first magnetostrictive bar; a second magnetostrictive bar extending alongside the first magnetostrictive bar; a joint member coupling the first magnetostrictive bar and the second magnetostrictive bars; and a coil group including a first coil wound around the first magnetostrictive bar and a second coil wound around the second magnetostrictive bar, wherein the first coil and the second coil are connected in series. 
         [0011]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0012]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]      FIG. 1  is a front view of a power generating device used in studies; 
           [0014]      FIG. 2  is a side view of the power generating device used in the studies; 
           [0015]      FIG. 3  is a schematic diagram for explaining the principle of power generation by the power generating device used in the studies; 
           [0016]      FIG. 4  is a graph schematically illustrating temporal changes in impedance of coils of the power generating device used in the studies; 
           [0017]      FIG. 5  is a Smith chart illustrating impedance of the coils of the power generating device used in the studies; 
           [0018]      FIG. 6  is a front view of a power generating device according to a first embodiment; 
           [0019]      FIG. 7  is a side view of the power generating device according to the first embodiment; 
           [0020]      FIG. 8  is an equivalent circuit diagram of the power generating device according to the first embodiment; 
           [0021]      FIG. 9  is a graph schematically illustrating a temporal change in combined impedance of a coil group of the power generating device according to the first embodiment; 
           [0022]      FIG. 10  is a Smith chart illustrating combined impedance of the power generating device according to the first embodiment; 
           [0023]      FIG. 11  is a front view of a power generating device according to a second embodiment; 
           [0024]      FIG. 12  is an equivalent circuit diagram of the power generating device according to the second embodiment; 
           [0025]      FIG. 13  is a Smith chart illustrating combined impedance of a power supply of the power generating device according to the second embodiment; 
           [0026]      FIG. 14  is a front view of a power generating device according to a third embodiment; 
           [0027]      FIGS. 15A and 15B  are equivalent circuit diagrams of the power generating device according to the third embodiment. 
           [0028]      FIG. 16  is a front view of a power generating device according to a fourth embodiment; 
           [0029]      FIG. 17  is a side view of one of power generating elements included in the power generating device according to the fourth embodiment; 
           [0030]      FIG. 18  is a schematic diagram for explaining the principle of power generation by the power generating device according to the fourth embodiment; 
           [0031]      FIG. 19  is an equivalent circuit diagram of the power generating device according to the fourth embodiment; 
           [0032]      FIG. 20  is a graph schematically illustrating temporal change in combined impedance of a coil group of the power generating device according to the fourth embodiment; and 
           [0033]      FIG. 21  is a block diagram of a sensor system according to a fifth embodiment. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0034]    Prior to description of embodiments, the studies conducted by the inventor are described. 
         [0035]      FIG. 1  is a front view of a power generating device used in the studies. 
         [0036]    The power generating device  10  is configured to generate electric power by applying stress to a magnetostrictive material. The power generating device  10  includes first and second magnetostrictive bars  1  and  2 , first and second coils  3  and  4 , and first and second joint members  5  and  6 . 
         [0037]    Each of the magnetostrictive bars  1  and  2  is made of a magnetostrictive material such as iron gallium alloy, for example, and the length thereof is about 10 mm. Note that iron gallium alloy is sometimes called Galfenol. The cross-section of each of the magnetostrictive bars  1  and  2  has a rectangular profile, the long and short sides of the rectangular are about 1 mm and about 0.5 mm, respectively. 
         [0038]    The magnetostrictive bars  1  and  2  extend alongside each other. Ends of the magnetostrictive bars  1  and  2  on one side are coupled to each other with the first joint member  5 , while ends of the magnetostrictive bars  1  and  2  on the other side are coupled to each other with the second joint member  6 . The joint members  5  and  6  are made of a magnetic material including iron, and are mechanically and magnetically connected to the magnetostrictive bars  1  and  2 . 
         [0039]    The first coil  3  is wound around the outer circumference of the first magnetostrictive bar  1 , and the second coil  4  is wound around the outer circumference of the second magnetostrictive bar  2 . These coils  3  and  4  are copper wire, for example. Each of the coils  3  and  4  has about 300 turns. 
         [0040]      FIG. 2  is a side view of the power generating device  10 . 
         [0041]    The same components in  FIG. 2  as those described in  FIG. 1  are given the same reference numerals as those described in  FIG. 1 , and the description thereof is omitted. 
         [0042]    As illustrated in  FIG. 2 , first and second permanent magnets  8  and  9  are magnetically and mechanically connected to both ends of the first magnetostrictive bar  1 , respectively. Note that the permanent magnets  8  and  9  are also magnetically and mechanically connected to both ends of the second magnetostrictive bar  2  (see  FIG. 1 ) as well as the first magnetostrictive bar  1 . 
         [0043]    A yoke  7  is provided beside the magnetostrictive bars  1  and  2 , in such a way that the yoke  7  is extending alongside the magnetostrictive bars  1  and  2 . The yoke  7  is magnetically and mechanically connected to the permanent magnets  8  and  9 . The material of the yoke  7 , which is not particularly limited, is a magnetic material including iron in this example. 
         [0044]    In the power generating device  10 , the bars  1  and  2  and yoke  7  form a magnetic path. The magnetic field H generated by the permanent magnets  8  and  9  circulates along this magnetic path. 
         [0045]    Because of the existence of the magnetic field H, the easy axis of magnetization of the magnetostrictive material of the magnetostrictive bars  1  and  2  is directed along the axial direction of the magnetostrictive bars  1  and  2 . This is also the case for the embodiments described later. 
         [0046]      FIG. 3  is a schematic view for explaining the principle of power generation by the power generating device  10 . In  FIG. 3 , the same components as those described in  FIGS. 1 and 2  are given the same reference numerals as those in  FIGS. 1 and 2 , and the description thereof is omitted below. 
         [0047]    As illustrated in  FIG. 3 , in actual use, the power generating device  10  is fixed to a vibrating body  13 , such as a bridge or a motor. In this example, the first joint member  5  is fixed to the vibrating body  13 , for example. 
         [0048]    Vibration of the vibrating body  13  causes vibration of the first and second magnetostrictive bars  1  and  2 , which causes periodic expansion and contraction of these magnetostrictive bars  1  and  2 . In this example, the both ends of the magnetostrictive bar  1  and  2  are coupled to each other with the joint members  5  and  6  as described above. Accordingly, the magnetostrictive bars  1  and  2  do not vibrate independently. Rather, the magnetostrictive bars  1  and  2  expand and contract in directions A and B opposite to each other. 
         [0049]    With such expanding and contracting motion, an inverse magnetostrictive effect which changes the magnetization of the magnetostrictive material is induced in each of the magnetostrictive bars  1  and  2 . Therefore, the magnetic flux penetrating each of the coils  3  and  4  fluctuates with time, so that the induced electromotive force can be extracted from the coils  3  and  4 . 
         [0050]    This induced electromotive force may be extracted from the first and second coils  3  and  4  individually. Alternatively, the induced electromotive force may be extracted from the first and second coils  3  and  4  which are connected in parallel. 
         [0051]    It is therefore considered that the energy harvesting technique can be implemented by converting the vibration of the vibrating body  13  into electric power with the power generating device  10  in this manner. 
         [0052]    However, the power generating device  10  has a problem that it is difficult to make impedance matching with another circuit. The problem is described below. 
         [0053]      FIG. 4  is a graph schematically illustrating temporal changes in impedance of the first and second coils  3  and  4 . The horizontal axis of the graph represents time, while the vertical axis represents absolute values of impedance. 
         [0054]    When the magnetostrictive bars  1  and  2  expand and contract along with the vibration of the vibrating body  13  as described above, the magnetization of the magnetostrictive bars  1  and  2  changes, and hence the magnetic permeability of the magnetostrictive bars  1  and  2  changes with time. Accordingly, impedances of the first and second coils  3  and  4 , which are respectively wound around the magnetostrictive bars  1  and  2 , change at the same cycle as that of the vibration of the magnetostrictive bars  1  and  2 . 
         [0055]    In this example, the directions A and B of the contraction of the first and second magnetostrictive bars  1  and  2  are opposite to each other. Therefore, the temporal changes in impedance of the first and second coils  3  and  4  have opposite phases to each other. 
         [0056]    Because of the changes in impedance described above, when the power generating device  10  is connected to another circuit, the impedance of the power generating device  10  seen from the circuit changes with time, so that impedance matching between the circuit and the power generating device  10  cannot be achieved. Accordingly, electric power from the power generating device  10  is not efficiently transmitted to another circuit, which results in energy loss. 
         [0057]      FIG. 5  is a Smith chart illustrating the impedances of the first and second coils  3  and  4  described above. 
         [0058]    In this example, it is assumed that a downward force of 1.2 kgf is applied to the second joint member  6 , and the frequency of the electromotive force induced by each coil  3  or  4  is assumed to range from 80 to 120 Hz. 
         [0059]    As illustrated in  FIG. 5 , when neither the first magnetostrictive bar  1  nor second magnetostrictive bar  2  expands or contracts, an impedance Z 0  of each of the first and second coils  3  and  4  is located in the upper half-plane of the Smith chart. 
         [0060]    When the magnetostrictive bars  1  and  2  expand and contract, impedance Z 1  of the first coil  3  and impedance Z 2  of the second coil  4  move on a constant resistance circle in directions opposite to each other. This is because the magnetostrictive bars  1  and  2  expand and contract in directions opposite to each other as described above, and the magnetization of one of the magnetostrictive bars  1  and  2  increases while the magnetization of the other one decreases. 
         [0061]    Note that the values that the impedances Z 0 , Z 1 , and Z 2  can take draw an arc on the constant-resistance circle. This is because the impedances have different values depending on the frequency of the induced electromotive force induced in the coils  3  and  4 . 
         [0062]    Even when the magnetostrictive bars  1  and  2  expand and contract in this manner, the impedance Z 1  and impedance Z 2  are located in the upper half-plane of the Smith chart and are not located on the real axis. 
         [0063]    Impedance matching between a power supply and a circuit is often performed at 50Ω on the real axis. Accordingly, when the impedances Z 1  and Z 2  are located in the upper half-plane in this manner, it becomes more difficult to achieve impedance matching between the power generating device  1  and another circuit. 
         [0064]    Hereinafter, a description is given of the embodiments in which impedance matching with another circuit can be easily achieved and electric power can be efficiently extracted through the another circuit. 
       First Embodiment 
       [0065]      FIG. 6  is a front view of a power generating device according to a first embodiment. In  FIG. 6 , the same components as those described in  FIG. 1  are given the same reference numerals as those in  FIG. 1 , and the description thereof is omitted. 
         [0066]    As illustrated in  FIG. 6 , a power generating device  20  is used by fixing it to a vibrating body  13  such as a bridge, and includes first and second magnetostrictive bars  1  and  2 . 
         [0067]    The both ends of the magnetostrictive bars  1  and  2  are coupled to each other with joint members  5  and  6 . Also, there is provided first and second coils  3  and  4  around the magnetostrictive bars  1  and  2  respectively. Each of the first and second coils  3  and  4  has about 300 turns. 
         [0068]    As described with reference to  FIG. 1 , the magnetostrictive bars  1  and  2  may be made of a magnetostrictive material such as iron gallium alloy, and the joint members  5  and  6  may be made of a magnetic materials including iron. 
         [0069]    The magnetostrictive bars  1  and  2  have a length of about 10 mm, and the cross-section thereof has a rectangular profile. The long and short sides of the rectangular are about 1 mm and about 0.5 mm, respectively. 
         [0070]      FIG. 7  is a side view of the power generating device  20 . 
         [0071]    As illustrated in  FIG. 7 , first and second permanent magnets  8  and  9  are connected to both ends of the magnetostrictive bars  1  and  2 , respectively. The permanent magnets  8  and  9  are connected to a yoke  7 , which forms a magnetic path in cooperation with the magnetostrictive bars  1  and  2 . The magnetic field H generated by the permanent magnets  8  and  9  circulates along this magnetic path. 
         [0072]    In the power generating device  20 , as in the example of  FIG. 3 , when the vibrating body  13  vibrates, the magnetostrictive bars  1  and  2  expand and contract, and the magnetic flux penetrating each of the coils  3  and  4  thereby changes with time, so that induced electromotive force is produced in the coils  3  and  4 . 
         [0073]      FIG. 8  is an equivalent circuit diagram of the power generating device  20 . 
         [0074]    As illustrated in  FIG. 8 , in the present embodiment, the first and second coils  3  and  4  are connected in series and constitute a coil group  21 . The coil group  21  is connected to a capacitor C and a resistor R in series. Note that the resistance value of the resistor R includes values of internal resistances of the coils  3  and  4 . 
         [0075]    In the power generating device  20 , the portions where the capacitor C and resistor R are provided are not particularly limited. The capacitor C and resistor R may be provided at the arbitrary portions of the power generating device  20 . This is also the case for the embodiments described later. 
         [0076]    The power generating device  20  includes output terminals  20   a  and  20   b , and the induced electromotive force produced in the first and second coils  3  and  4  is extracted through the terminals  20   a  and  20   b.    
         [0077]      FIG. 9  is a graph schematically illustrating temporal change in combined impedance of the coil group  21 , and the horizontal axis represents time while the vertical axis represents absolute values of the combined impedance. 
         [0078]    Note that  FIG. 9  also illustrates impedances of the first and second coils  3  and  4 . 
         [0079]    The combined impedance of the coil group  21  is the sum of the impedances of the coils  3  and  4 . However, as described with reference to  FIG. 4 , temporal changes in impedance of the first and second coils  3  and  4  have opposite phases. Therefore, the combined impedance of the coil group  21  is constant in time. 
         [0080]    As such, once the impedance matching between the power generating device  20  and another circuit is achieved at a certain time, impedance matching can be kept achieved in the subsequent arbitrary time, thereby always enabling efficient transmission of electric power from the power generating device  20  to another circuit. 
         [0081]      FIG. 10  is a Smith chart illustrating a combined impedance Z of the power generating device  20 . 
         [0082]    In this example, it is assumed that a downward force of 1.2 kgf is applied to the second joint member  6  of the power generating device  20 , and the frequency of the electromotive force induced in the coil  3  and  4  is assumed to range from 80 Hz to 120 Hz. 
         [0083]    Note that  FIG. 10  also illustrates the impedances Z 0 , Z 1 , and Z 2  of  FIG. 5 . As described with reference to  FIG. 5 , the impedance Z 0  is the impedance of each of the coils  3  and  4  when the magnetostrictive bars  1  and  2  are not expanding and contracting. The impedances Z 1  and Z 2  are impedances of the coils  3  and  4  respectively when the magnetostrictive bars  1  and  2  are expanding and contracting. 
         [0084]    As illustrated in  FIG. 10 , since the resistance R is added to the coil group  21  as described above, the combined impedance Z of the power generating device  20  of the present embodiment is located on a constant-resistance circle of higher resistance than that of the aforementioned impedances Z 0 , Z 1 , and Z 2 . 
         [0085]    Moreover, because of the capacitor C added to the coil group  21 , the combined impedance Z moves toward the real axis along the constant-resistance circle. Impedance matching with external circuits is often performed on the real axis. Accordingly, when the combined impedance Z becomes close to the real axis as in the above, the impedance matching between the power generating device  20  and another circuit is easily achieved. 
         [0086]    Note that the possible values of the combined impedance Z is seen like an arc on the constant resistance circle. This is because the combined impedance Z has different values depending on the frequency of the electromotive force induced by the coil group  21 . This is also the case for the second embodiment described later. 
         [0087]    Moreover, the combined impedance Z may be located on the real axis by adjusting the capacitance value of the capacitor C in the following manner so that the reactance component of the combined impedance Z is 0. 
         [0088]    Since the equivalent circuit of  FIG. 8  is a series RLC circuit, the combined impedance z of the power generating device  20  is expressed by the following equation (1): 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       
                         Z 
                         = 
                           
                          
                         
                           R 
                           + 
                           
                             j 
                              
                             
                                 
                             
                              
                             ω 
                              
                             
                                 
                             
                              
                             L 
                           
                           + 
                           
                             1 
                             
                               j 
                                
                               
                                   
                               
                                
                               ω 
                                
                               
                                   
                               
                                
                               C 
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           R 
                           + 
                           
                             j 
                              
                             
                                 
                             
                              
                             ω 
                              
                             
                                 
                             
                              
                             L 
                           
                           - 
                           
                             j 
                              
                             
                                 
                             
                              
                             
                               1 
                               
                                 ω 
                                  
                                 
                                     
                                 
                                  
                                 C 
                               
                             
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                          
                         
                           R 
                           + 
                           
                             
                               j 
                                
                               
                                 ( 
                                 
                                   
                                     ω 
                                      
                                     
                                         
                                     
                                      
                                     L 
                                   
                                   - 
                                   
                                     1 
                                     
                                       ω 
                                        
                                       
                                           
                                       
                                        
                                       C 
                                     
                                   
                                 
                                 ) 
                               
                             
                             . 
                           
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0089]    In the equation (1), the resistance value of the resistor R and the capacitance value of the capacitor C are indicated by the same letters R and C as those of the resistor R and capacitor C, respectively. Moreover, L is the combined inductance of the first and second coils  3  and  4 , and ω is an angular frequency of vibration of the magnetostrictive bars  1  and  2 . 
         [0090]    In order to make the reactance component of the combined impedance to 0, the imaginary part of the second term in the right-hand side of the equation (1) needs to be 0. Therefore, the reactance component of the combined impedance Z can be adjusted to 0 by controlling the capacitance of the capacitor C so as to satisfy ωL=1/(ωC). 
         [0091]    Accordingly, it is possible to easily achieve impedance matching between the power generating device  20  and another circuit, which is often performed on the real axis. 
         [0092]    Moreover, impedance matching is often performed at the point of 50Ω on the real axis as described above. When the reactance component of the combined impedance Z is 0, the combined impedance Z moves leftward on the real axis as the resistance R decreases, and the combined impedance z moves rightward on the real axis as the resistance R increases. 
         [0093]    Therefore, the combined impedance Z can be located at the point of 50Ω on the real axis by properly adjusting the resistance value of the resistor R. 
         [0094]    According to the present embodiment described above, the first and second coils  3  and  4  are connected in series as illustrated in  FIG. 8 . Therefore, the temporal changes in the impedances of the coils  3  and  4  can be canceled each other, so that the combined impedance of the power generating device  20  can be made constant in time. 
         [0095]    Moreover, since the capacitor C is connected to the coil group  21  composed of the coils  3  and  4  in series, the reactance component of the combined impedance of the power generating device  20  can be set to 0, so that impedance matching between the power generating device  20  and another circuit can be achieved on the real axis. 
         [0096]    Furthermore, by controlling the resistance value of the resistor R added to the coil group  21 , it is possible to achieve impedance matching at the point of 50Ω on the real axis. 
       Second Embodiment 
       [0097]      FIG. 11  is a front view of a power generating device according to a second embodiment. Note that the same components in  FIG. 11  as those described in the first embodiment are given the same reference numerals as those of the first embodiment, and the description thereof is omitted. 
         [0098]    As illustrated in  FIG. 11 , a power generating device  3 D includes a plurality of small regions  31  in each of first and second magnetostrictive bars  1  and  2 . Each of the small regions  31  are separated in the axis direction of the first and second magnetostrictive bars  1  and  2 . 
         [0099]    In each of the small regions  31 , one of the coils  3  and  4  is provided. Thus, the first magnetostrictive bar  1  is provided with the plural first coils  3 , while the second magnetostrictive bar  2  is provided with plural second coils  4 . 
         [0100]    Moreover, the coil group  21  is formed by connecting one first coil  3  and one second coil  4  in series. In the present embodiment, a plurality of coil group  21  is provided. 
         [0101]      FIG. 12  is an equivalent circuit diagram of the power generating device  30  according to the second embodiment. 
         [0102]    As illustrated in  FIG. 12 , in the power generating device  30 , each of the plural coil groups  21  is provided with output terminals  30   a  and  30   b . The induced electromotive force of each coil group  21  is extracted through the output terminals  30   a  and  30   b.    
         [0103]    By providing the plural coil groups  21  in this manner, it is possible to obtain as many independent power supplies  21   a  as the number of the coil groups  21 . 
         [0104]    Moreover, in order to adjust the combined impedance of each power supply  21   a , each coil group  21  is connected to a capacitor C and a resistor R in series as in the first embodiment. Note that the resistance R includes internal resistance of the corresponding coil group  21  as described in the first embodiment. 
         [0105]      FIG. 13  is a Smith chart illustrating combined impedance Z 3  of one of the power supplies  21   a.    
         [0106]    In this example, it is assumed that a downward force of 1.2 kgf is applied to the second joint member  6  of the power generating device  30 , and the frequency of the electromotive force induced by each of the coils  3  and  4  is assumed to range from 80 Hz to 120 Hz. 
         [0107]      FIG. 13  also illustrates the impedances Z 0 , Z 1 , and Z 2  of  FIG. 5 . 
         [0108]    In the present embodiment, each of the magnetostrictive bars  1  and  2  is divided into the small regions  31  as described above. Accordingly, the coils  3  and  4  provided for each small region  31  have lengths shorter than those in the first embodiment, and hence the resistances of the coils  3  and  4  are reduced. 
         [0109]    Accordingly, as indicated by an arrow F in  FIG. 13 , the combined impedance Z 3  of the power supply  21   a  moves more leftward on the real axis than in the first embodiment, so that the impedance Z 3  can be easily brought close to 50Ω. 
         [0110]    In such a manner, according to the present embodiment, since the internal resistances of the coils  3  and  4  are reduced, the combined impedance Z of the power supply  21   a  can be easily adjusted to 50Ω. 
       Third Embodiment 
       [0111]    In the second embodiment, the power supplies  21   a  are individually used as an independent power supply as illustrated in  FIG. 12 . In the present embodiment, these power supplies  21   a  are used in combination. 
         [0112]      FIG. 14  is a front view of a power generating device according to the present embodiment. The same components in  FIG. 14  as those described in the first and second embodiment are given the same reference numerals as those of these embodiments, and the description thereof is omitted. 
         [0113]    As illustrated in  FIG. 14 , in a power generating device  40  according to the present embodiment, a plurality of first coils  3  is provided for a first magnetostrictive bar  1 , and a plurality of second coils  4  is provided for a second magnetostrictive bar  2 , as in the second embodiment. 
         [0114]    Moreover, coil group  21  is constructed from one first coil  3  and one second coil  4  which are connected in series. 
         [0115]      FIGS. 15A and 15B  are equivalent circuit diagrams of the power generating device  40 . Note that the same components in  FIGS. 15A and 15B  as those described in  FIG. 12  are given the same reference numerals as those in  FIG. 12 , and the description thereof is omitted. 
         [0116]    In each example of  FIGS. 15A and 15B , the plural coil groups  21  are connected in parallel. Note that a plurality of capacitors C for adjusting the impedance is provided corresponding to each coil groups  21  in  FIG. 15A , whereas only the single capacitor C is provided in  FIG. 15B . 
         [0117]    Any one of constitutions of  FIGS. 15A and 15B  can be selected, based on the specifications of the power generating device  40  and the like. 
         [0118]    By connecting the coil groups  21  in parallel in this manner, the resistance of the power generating device  40  is reduced compared with the case where the coil groups  21  are used independently. Therefore, as in the example of  FIG. 13 , the combined impedance Z 3  of the power generating device  40  moves leftward on the real axis, so that the combined impedance Z 3  can be easily adjusted to 50Ω. 
         [0119]    Moreover, by adjusting the length of the coils  3  and  4  to control the internal resistances of the coils  3  and  4 , the combined impedance of the power generating device  40  can be easily adjusted along the real axis. 
       Fourth Embodiment 
       [0120]    In the first to third embodiments, the first coil  3  and second coil  4  in a generating device are connected in series. 
         [0121]    On the other hand, in the present embodiment, a plurality of the power generating elements are used in the following manner, and the coils of each of the power generating elements are connected in series. 
         [0122]      FIG. 16  is a front view of a power generating device according to the present embodiment. 
         [0123]    The power generating device  50  includes two power generating elements  51 . 
         [0124]    The power generating elements  51  are fixed to a vibrating body  13  such as a bridge. Each power generating element  51  includes a first bar  52  made of a magnetostrictive material and a second bar  53  extending alongside the first bar  52 . 
         [0125]    Note that the second bar  53  is disposed away from the first bar  52  in a direction D 1 . The first directions D 1  in the two power generating elements  51  are opposite to each other. 
         [0126]    One ends of the first and second bars  52  and  53  are coupled to each other with a first joint member  5 , while the other ends of the first and second bars  52  and  53  are coupled to each other with a second joint member  6 . As in the first to third embodiments, the joint members  5  and  6  are made of a magnetic material including iron and are connected to the bars  52  and  53  mechanically and magnetically. 
         [0127]    Note that the material of the second bar  53  does not need to be a magnetostrictive material and may be a magnetic material such as iron. 
         [0128]    Moreover, coils  55  such as a copper wire or the like are wound around the respective first bars  52 . The coils  55  of the plural power generating elements  51  are connected to each other in series to constitute a coil group  57 . 
         [0129]      FIG. 17  is a side view of the power generating device  51 . 
         [0130]    Note that the same components described in  FIG. 17  as those described in the first to third embodiments are given the same reference numerals as these embodiments, and the description thereof is omitted. 
         [0131]    As illustrated in  FIG. 17 , first and second permanent magnets  8  and  9  are mechanically and magnetically connected to both ends of the first bar  52 , respectively. 
         [0132]    Moreover, a yoke  7  is provided beside the first bar  52 . The yoke  7  extends alongside the first bar  52 , and is magnetically and mechanically connected to the permanent magnets  8  and  9 . The material of the yoke  7  is a magnetic material including iron, for example. 
         [0133]    In this power generating element  51 , the bars  52  and  53  and yoke  7  form a magnetic path, and the magnetic field H generated by the permanent magnets  8  and  9  circulates along the magnetic path. 
         [0134]      FIG. 18  is a schematic diagram for explaining the principle of power generation by the power generating device  50 . Note that in  FIG. 18 , the same components as those described in  FIGS. 16 and 17  are given the same reference numerals as in the  FIGS. 16 and 17 , and the description thereof is omitted. 
         [0135]    In  FIG. 18 , the coils  55  are not illustrated in order to facilitate the visualization. 
         [0136]    As illustrated in  FIG. 18 , when the vibrating body  13  vibrates, the first and second bars  52  and  53  in each of the two power generating elements  51  vibrate, which causes expansion and contraction of these bars  52  and  53 . Thus, the magnetic flux penetrating the coil  55  wound around each first bar  52  changes in time, so that the induced electromotive force is produced in the coil  55 . 
         [0137]    Here, when each of the power generating elements  51  vibrate in a substantially identical phase by the vibration of the vibrating body  13 , the first bar  52  in one of the power generating elements  51  contracts in the direction of arrow A, while the first bar  52  in the other power generating element  51  expands in the direction of arrow B. This is because the first directions D 1  of the power generating elements  51  are opposite to each other as described above. Thus, the first bars  52  in the two power generating elements  51  expand and contract in the opposite phases to each other. 
         [0138]      FIG. 19  is an equivalent circuit diagram of the power generating device  50 . 
         [0139]    As illustrated in  FIG. 19 , the coils  55  of the power generating devices  51  are connected in series as described above, and the coils  55  constitute a coil group  57 . 
         [0140]    Moreover, the coil group  57  is connected in series to a capacitor C and a resistance R for adjusting the impedance of the power generating device  50 . Note that the resistance value of the resistance R includes the internal resistance value of the coils  55 . 
         [0141]    Moreover, the power generating device  50  is provided with output terminals  50   a  and  50   b . The induced electromotive force produced in the coil group  57  is extracted through the terminals  50   a  and  50   b.    
         [0142]      FIG. 20  is a graph schematically illustrating temporal change in the combined impedance of the coil group  57 . The horizontal axis of the graph represents time, while the vertical axis represents absolute values of the combined impedance. 
         [0143]      FIG. 20  also illustrates impedances of the coils  55  of the two power generating elements  51 . 
         [0144]    As described above, the directions in which the first bars  51  expand and contract is opposite in the two power generating elements  51 . Accordingly, the temporal changes in impedance of the coils  55  becomes opposite phases in the two power generating elements  51 . 
         [0145]    As the result, the combined impedance of the coil group  57 , which is represented by the sum of impedances of the coils  55 , becomes constant in time. 
         [0146]    In the present embodiment, as in the first to third embodiments, the combined impedance of the power generating device  50  is constant in time, so that impedance matching between the power generating device  50  and another circuit can be achieved. 
         [0147]    Moreover, by connecting the capacitor C and resistor R for impedance adjustment to the coil group  57  as illustrated in  FIG. 19 , impedance matching between the power generating device  50  and another circuit can be achieved on the real axis because of the same reason as the first embodiment. 
       Fifth Embodiment 
       [0148]    In the present embodiment, a description is given of a sensor system using the power generating device described in the first to fourth embodiments. 
         [0149]      FIG. 21  is a block diagram of the sensor system according to the present embodiment. 
         [0150]    A sensor system  60  monitors a bridge or the like by using electric power obtained by an energy harvesting technique. The sensor system  60  includes the power generating device  20  described in the first embodiment, and a circuit unit  61  supplied with electric power from the power generating device  20 . 
         [0151]    Note that instead of the power generating device  20 , the power generating devices  30 ,  40 , and  50  described in the second to fourth embodiments may be used. 
         [0152]    The circuit unit  61  includes a boost rectifier  62 , a secondary battery  63 , a sensor  64 , a microcomputer  65 , and a communication unit  66 . 
         [0153]    The boost rectifier  62  rectifies the output current of the power generating device  20 . The boost rectifier  62  boosts the output voltage of the power generating device  20  to a predetermined DC voltage. 
         [0154]    The secondary battery  63  is charged with the above DC voltage. Then, the sensor  64 , microcomputer  65 , and communication unit  66  are driven with the output voltage of the secondary battery  63 . 
         [0155]    The sensor  64  is an acceleration sensor to detect vibration of a bridge, a tunnel, and the like, for example. The sensor  64  sends a sensor signal S including the magnitude and cycle of the vibration to the microcomputer  65 . Instead of the acceleration sensor, the sensor  64  may be a temperature sensor or a pressure sensor to measure the temperature or pressure of the atmosphere. 
         [0156]    The frequency at which the sensor signal S is acquired is controlled by the microcomputer  65 . The sensor signal S is acquired at a frequency of once a day, for example. 
         [0157]    The microcomputer  65  sends the sensor signal S to the communication unit  66 . The communication unit  66  wirelessly transmits the sensor signal S to the outside according to a predetermined wireless protocol. 
         [0158]    The sensor signal S wirelessly transmitted is received by a terminal such as a personal computer, for example. By using the terminal, a user can see information of the environment where the sensor  64  is placed. 
         [0159]    According to the sensor system  60 , as described in the first embodiment, the impedance of the power generating device  20  is constant in time. Therefore, the state where the impedance matching between the power generating device  20  and the circuit unit  61  is achieved can be maintained, thus preventing loss of electric power supplied from the power generating device  20  to the circuit unit  61 . 
         [0160]    All examples and conditional language recited herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.