BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates an acceleration sensor including a piezoelectric element. 
     2. Description of the Related Art 
     Japanese Unexamined Patent Application Publication No. 6-273439 (corresponding to counterpart U.S. Pat. Nos. 5,515,725 and 5,490,422) discloses an acceleration sensor including a piezoelectric ceramic. Each of two piezoelectric ceramic layers, bonded together, includes three longitudinally aligned regions at two borders at which stress is inverted in the longitudinal direction of the layers when acceleration is applied thereto. These three regions are connected in parallel, and then the two layers are connected in series. FIG. 8A illustrates a charge that is generated when an acceleration G acts on the acceleration sensor, and FIG. 8B is a circuit diagram of the acceleration sensor. 
     Japanese Unexamined Patent Application Publication No. 2000-121661 (corresponding to counterpart U.S. Pat. No. 6,360,603) discloses another acceleration sensor. Each of two piezoelectric ceramic layers, bonded together, includes three longitudinally aligned regions separated at two borders at which stress is inverted in the longitudinal direction of the layers when acceleration is applied thereto. These three regions are connected in parallel, and then the two layers are connected in parallel. FIG. 9A illustrates a charge that is generated when an acceleration G acts on the acceleration sensor, and FIG. 9B is a circuit diagram of the acceleration sensor. 
     In the acceleration sensor shown in FIGS. 8A and 8B, cells ( 1 ) through ( 3 ) in one layer are connected in parallel and cells ( 4 ) through ( 6 ) in the other layer are connected in parallel. In the acceleration sensor shown in FIGS. 9A and 9B, cells ( 1 ) through ( 6 ) in the two layers are connected in parallel. In these arrangements, voltages generated in the cells are not summed. Sensitivity to the voltage generated in each layer is low, and the performance of these acceleration sensors is not high enough in applications where high sensitivity is required. Since all six cells ( 1 ) through ( 6 ) are connected in parallel in the acceleration sensor shown in FIGS. 9A and 9B, the voltage sensitivity thereof is half the voltage sensitivity provided by the acceleration sensor shown in FIGS. 8A and 8B. 
     To increase the voltage sensitivity, the gain of a voltage amplifier connected to the sensor must be increased. If the gain of the amplifier is increased, a noise component applied to the amplifier is also amplified. Also, the S/N ratio is degraded. Increasing the gain of the voltage amplifier does not serve the purpose of measuring a small acceleration. 
     In an acceleration sensor including a piezoelectric ceramic, no noise is generated from the piezoelectric ceramic itself. The sources of noise include external noise ingressing between the sensor and the input terminal of a circuit thereof, and internal noise responsive to input conversion voltage noise generated at an input stage of an operational amplifier defining the amplifier. 
     These noises will now be discussed with reference to a charge amplifier shown in FIG. 10 and a voltage amplifier shown in FIG.  11 . 
     The sensor S here has voltage sensitivity Vs, charge sensitivity Qs, and capacitance Cs. It is well known that the relationship of Qs=Vs×Cs holds. Overall gains G of the two amplifiers are listed in Table 1. In the charge amplifier, the gain G is proportional to the charge sensitivity Qs. In the voltage amplifier, the gain is proportional to the voltage sensitivity Vs. 
     
       
         
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 External 
                 Internal 
                 G converted 
                 G converted 
               
               
                   
                   
                 noise 
                 noise 
                 external 
                 internal 
               
               
                 Circuit 
                   
                 voltage 
                 voltage 
                 noise 
                 noise 
               
               
                 type 
                 Gain G 
                 (x Vn) 
                 (x En) 
                 (x Vn) 
                 (x En) 
               
               
                   
               
             
             
               
                 Charge 
                 Qs/C1 
                 Cc/C1 
                 (Cs + C1)/C1 
                 Cc/Qs 
                 (Cs + C1)/Qs = 
               
               
                 amplifier 
                   
                   
                   
                   
                 {(Cs + C1)/Cs}/Vs 
               
               
                 Voltage 
                 Vs = Qs/Cs 
                 Cc/Cs 
                 1 
                 Cc/Qs 
                 1/Vs 
               
               
                 amplifier 
               
               
                   
               
             
          
         
       
     
     The external noise Vn is now considered. The external noise Vn is electrostatically coupled through capacitance Cc to a line running between a sensor S and an input terminal of an amplifier OP, and the magnitude of noise is thus represented by Vn. The external noise Vn appears as Von after being amplified through the amplifier. Since the polarity of noise is not important, all noises are expressed in absolute values in Table 1. 
     In the charge amplifier, a coupling capacitance Cc and a feedback capacitance C1 form an inverting amplifier. The external noise voltage has a magnitude that is obtained by multiplying Vn by gain (Cc/C1). In the voltage amplifier, noise is voltage divided by a coupling capacitance Cc and capacitance Cs of a sensor, and is then output through a voltage follower. 
     If G converted noise that represents the magnitude of noise with reference to the level G is defined by the reciprocal of the S/N ratio, the G converted noise becomes a noise voltage/G gain. For example, if the G converted noise is 100 mG, noise as large as 100 mG is continuously generated even if no acceleration appears in the output of the circuit. An acceleration below that level cannot be measured. The G converted noise in each circuit is listed in Table 1. The G converted external noise is not related to the circuit type. The larger the charge sensitivity Qs, the smaller the G converted external noise, and the smaller acceleration is measured. 
     Table 1 lists internal noise voltage. Internal noise En is generated at a positive input terminal of an operational amplifier OP. A non-inverting amplifier is defined by a capacitance Cs of the sensor S and the feedback capacitance C1, and the output thereof is obtained by multiplying En by the resulting gain. Since the voltage amplifier is a voltage follower of gain one, En is directly output. The G converted internal noise is obtained by dividing noise voltage Von by G gain Vos as listed in Table 1. The G converted internal noise is not related to the circuit type. The larger the voltage sensitivity Vs, the smaller the G converted internal noise, and the smaller acceleration that is measured. 
     A sensor having a large charge sensitivity and a large voltage sensitivity is a good sensor. The product Qs·Vs/2 of the charge sensitivity and voltage sensitivity corresponds to energy Es generated by acceleration. To increase the energy Es, the sensor must be large in size. The external noise here depends on the layout on a printed circuit board, and the internal noise depends on an amplifier characteristic of an operational amplifier in use. Depending on the status of a host apparatus of the sensor, more weight is attached to one of the charge sensitivity Qs and the voltage sensitivity Vs than the other while the energy Es, which is the product of the charge sensitivity and the voltage sensitivity, is maintained. The above-disclosed sensors with the large charge sensitivity thereof are appropriate for use in applications where external noise level is high, but are not appropriate for use in applications where internal noise level is high because of the small voltage sensitivity thereof. 
     SUMMARY OF THE INVENTION 
     In order to overcome the problems described above, preferred embodiments of the present invention provide an acceleration sensor which increases a voltage sensitivity without reducing energy, which is equal to the product of charge sensitivity and voltage sensitivity, as much as possible, and which is appropriate in an operating environment where internal noise is high in level. 
     An acceleration sensor of a preferred embodiment of the present invention includes a piezoelectric element and a support member for supporting the piezoelectric element at both longitudinal ends thereof. The piezoelectric element includes a laminate of at least two piezoelectric layers. Each of the least two external piezoelectric layers of the piezoelectric element includes three longitudinally aligned regions separated at two borders where stress is inverted in the longitudinal direction of the piezoelectric element when acceleration is applied. Each of a plurality of cells defined by a respective region is polarized in the same direction of thickness in each of the two external piezoelectric layers. Electrodes are arranged on the top and bottom major surfaces and between the layers of the piezoelectric element so that the three longitudinally aligned cells are electrically connected in series. 
     The acceleration sensor of preferred embodiments of the present invention including a serial electrical connection of the three cells in the at least two external piezoelectric layers in the longitudinal direction presents a voltage sensitivity that is relatively higher than a conventional sensor in which the cells are connected in parallel. Although a charge sensitivity decreases, the energy equal to the product of the charge sensitivity and the voltage sensitivity suffers from no significant change as in the conventional sensor. The voltage sensitivity is thus increased while a decrease in the generated energy is controlled. Thus, preferred embodiments of the present invention provide an acceleration sensor that is appropriate for use in applications where internal noise is high. 
     The three cells in the at least two external piezoelectric layers or six cells in the at least two piezoelectric layers are serially connected. If an insulation resistance between electrodes in any particular cell is degraded, the remaining cells maintain the performance thereof. The entire sensor is thus less sensitive to individual cell failure. The thickness of the piezoelectric layer is reduced and sensitivity is increased as the degree of required insulation is accordingly alleviated. 
     In the conventional acceleration sensor, three separate cells arranged in the longitudinal direction are inverted in polarization between two adjacent cells. To polarize the sensor, three external electrodes are arranged for three cells, voltages for the three cells for polarization are applied, and electrodes for connecting the external electrodes are then arranged. In the acceleration sensor of preferred embodiments of the present invention, three cells in each of the at least two piezoelectric layers are polarized in the same direction. There is no need for inverting the polarization direction in the longitudinal direction. In the production of the sensor, the electrodes are manufactured in their final shape. The polarization process and the electrode formation process are thus simplified. In addition, manufacturing costs are reduced. 
     The piezoelectric element of the present invention is not limited to a two-layer structure. The piezoelectric element may include two or more layers in the structure thereof. 
     The polarization direction and the electrode structure may have various particular conditions. Preferably, all cells of the two external piezoelectric layers are polarized in the same direction. Each of the electrodes on the top and bottom major surfaces of the piezoelectric element includes two longitudinally aligned portions separated at one of the two borders. The electrode between the piezoelectric layers of the piezoelectric element includes two longitudinally aligned portions separated at the other of the two borders. At least one of the electrodes on the top and bottom major surfaces is lead out to one of the longitudinal ends of the piezoelectric element, and the electrode between the piezoelectric layers is lead out to the other of the longitudinal ends of the piezoelectric element. 
     In this case, the three cells in each of the two layers are serially connected, and then, two layers are connected in parallel. This arrangement results in a voltage sensitivity which is about 1.5 times that of the conventional sensor shown in FIG. 8A, and about 3 times that of the conventional sensor shown in FIG.  9 A. Although this arrangement has a charge sensitivity which is about 0.6 times that of the conventional sensor shown in FIG. 8A, and about 0.3 times that of the conventional sensor shown in FIG. 9A, the energy remains substantially unchanged, namely, about 0.9 times that of the conventional sensor. The voltage sensitivity is increased without significantly reducing the energy. An acceleration sensor that is appropriate for use in applications where internal noise level is high is thus provided. Since the polarization directions of all cells are the same, the manufacturing process of the sensor is simplified, and the manufacturing costs are reduced. 
     The polarization direction and the electrode structure may have other particular conditions. Preferably, the cell of the one piezoelectric layer and the cell of the other piezoelectric layer are polarized in opposite directions. The electrode on the top major surface of the piezoelectric element preferably includes two longitudinally aligned portions separated at one of the two borders. The electrode on the bottom major surface of the piezoelectric element preferably includes two longitudinally aligned portions separated at the other of the two borders. The interlayer electrode of the piezoelectric element includes three longitudinally aligned regions separated at the two borders. The electrode on the top major surface is lead out to one of the longitudinal ends of the piezoelectric element, and the electrode on the bottom major surface is lead out to the other of the longitudinal ends. 
     In this arrangement, six cells in the at least two piezoelectric layers are serially connected. This arrangement results in a voltage sensitivity which is about 3 times that of the conventional sensor shown in FIG. 8A, and about 6 times that of the conventional sensor shown in FIG.  9 A. Although this arrangement has a charge sensitivity which is about 0.3 times that of the conventional sensor shown in FIG. 8A, and about 0.15 times that of the conventional sensor shown in FIG. 9A, the energy remains substantially unchanged, namely, about 0.9 times that of the conventional sensor. The voltage sensitivity is heightened without significantly reducing the energy. An acceleration sensor that is appropriate for use in applications where internal noise level is high is thus provided. 
     Since the polarization directions of the three cells in each layer are the same, the manufacturing process of the sensor is simplified, and the manufacturing costs are reduced. 
     Other features, elements, characteristics and advantages of the present invention will become more apparent from the following detailed description of preferred embodiments of the present invention with reference to the attached drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of the acceleration sensor in accordance with a first preferred embodiment of the present invention; 
     FIG. 2 is a front view of the acceleration sensor of FIG. 1; 
     FIG. 3 is a circuit diagram of the acceleration sensor of FIG. 1; 
     FIG. 4 illustrates operation of the acceleration sensor of FIG. 1 with acceleration G applied thereto; 
     FIG. 5 is a front view of the acceleration sensor in-accordance with a second preferred embodiment of the present invention; 
     FIG. 6 is a circuit diagram of the acceleration sensor of FIG. 5; 
     FIG. 7 illustrates operation of the acceleration sensor of FIG. 5 with acceleration G applied thereto; 
     FIG. 8A illustrates operation of a conventional acceleration sensor under acceleration; 
     FIG. 8B illustrates a circuit diagram of the conventional acceleration sensor; 
     FIG. 9A illustrates operation of another conventional acceleration sensor under acceleration; 
     FIG. 9B illustrates a circuit diagram of the conventional acceleration sensor; 
     FIG. 10 is a circuit diagram of a charge amplifier; and 
     FIG. 11 is a circuit diagram of a voltage amplifier. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     FIGS. 1 through 4 illustrate the acceleration sensor  1 A in accordance with a first preferred embodiment of the present invention. 
     The acceleration sensor  1 A preferably includes a piezoelectric element (detector element)  2  which is supported at both ends by a pair of substantially U-shaped support frames  10  and  11 . The support frames  10  and  11  are preferably made of an electrically insulative ceramic having a thermal expansion coefficient almost equal to that of the piezoelectric element  2 . The support frames  10  and  11  have spaces  10   a  and  11   a  within which the piezoelectric element  2  is deformed in response to acceleration G applied thereto. 
     The piezoelectric element  2  of the first preferred embodiment is manufactured preferably by laminating two thin, substantially rectangular piezoelectric layers  2   a  and  2   b  and by baking the laminate. Each layer may be as thin as or thinner than approximately 100 μm to increase charge sensitivity. Electrodes  3   a  and  3   b  are arranged between the layers  2   a  and  2   b  in the piezoelectric element  2 , and electrodes  5   a  and  5   b  and electrodes  6   a  and  6   b  are arranged on top and bottom major surfaces of the piezoelectric element  2 . Each of the two piezoelectric layers  2   a  and  2   b  is divided into three regions by two borders B 1  and B 2  at which stress in response to the application of the acceleration G is inverted. The regions respectively define six cells ( 1 ) through ( 6 ). All cells are preferably polarized in the same thickness direction. The polarization directions P are represented by solid arrow marks. 
     The interlayer electrodes  3   a  and  3   b  arranged in the longitudinal direction of the element are spaced from each other along the border B 2 . The interlayer electrode  3   a  continuously extends between the two cells ( 1 ) and ( 2 ) and the two cells ( 3 ) and ( 4 ). The interlayer electrode  3   a  is terminated with one end close to the left-hand end surface of the piezoelectric element  2 , and with the other end near the border B 2 . The interlayer electrode  3   b  is arranged between the cells ( 3 ) and ( 6 ), and extends and terminated with one end near the border B 2  and the other end routed to the right-hand end surface of the piezoelectric element  2 . 
     The electrodes  5   a  and  5   b  on the top major surface of the piezoelectric element  2  are separated from each other along the border B 1 , and the electrodes  6   a  and  6   b  on the bottom major surface of the piezoelectric element  2  are separated from each other along the border B 1 . The electrodes  5   a  and  6   a  extend with each end routed to the left-hand end surface of the piezoelectric element  2  and the other ends terminated near the border B 1 . The electrodes  5   b  and  6   b  extend with ends close to the border B 1  and with the other ends close to the right-hand end of the piezoelectric element  2 . In other words, the major surface electrodes  5   a  and  6   a  are respectively disposed on the cells ( 1 ) and ( 4 ), and the major surface electrodes  5   b  and  6   b  are respectively disposed on the cells ( 2 ) and ( 3 ) and the cells ( 5 ) and ( 6 ). 
     External electrodes  7  and  8  are disposed on the longitudinal end surfaces of the piezoelectric element  2  and longitudinal end surfaces of the support frames  10  and  11 . The external electrode  7  disposed on one end surface of the piezoelectric element  2  is electrically connected to the top major surface electrodes  5   a  and the bottom major surface electrode  6   a , and the external electrode  8  disposed on the other end surface of the piezoelectric element  2  is electrically connected to the interlayer electrode  3   b.    
     A circuit shown in FIG. 3 is formed by arranging the interlayer electrodes  3   a  and  3   b , top and bottom major surface electrodes  5   a ,  5   b ,  6   a  and  6   b , and external electrodes  7  and  8 . The three cells ( 1 ) through ( 3 ) arranged in the longitudinal direction are connected in series in the piezoelectric layer  2   a  and the three cells ( 4 ) through ( 6 ) arranged in the longitudinal direction are connected in series in the piezoelectric layer  2   b . Furthermore, a connection of the cells ( 1 ) through ( 3 ) in the layer  2   a  and a connection of the cells ( 4 ) through ( 6 ) in the layer  2   b  are electrically connected in parallel. 
     Referring to FIG. 3, the node of the cell ( 1 ) and cell ( 2 ) and the node of the cell ( 4 ) through ( 5 ) are connected through the interlayer electrode  3   a . Since the top and bottom piezoelectric layers  2   a  and  2   b  are symmetrical, the nodes have the same potential. Regardless of whether or not the nodes are connected, the circuit remains unchanged in characteristics thereof. 
     FIG. 4 illustrates a charge that is generated when acceleration G acts on the acceleration sensor  1 A. 
     If the acceleration G works in a downward direction as represented by an arrow shown in FIG. 4, a central portion of the piezoelectric element  2  is deformed to be convex in an upward direction through inertia. A tensile stress works on the central cell ( 2 ) of the top piezoelectric layer  2   a , while a compressive stress works on the side cells ( 1 ) and ( 3 ). On the other hand, a compressive stress works on the central cell ( 5 ) in the bottom piezoelectric layer  2   b , while a tensile stress works on the side cells ( 4 ) and ( 6 ). Depending on the relationship between the above-mentioned stress and the polarization direction P, a positive charge is generated in the one top major surface electrode  5   a  and a negative charge is generated in one portion of the other top major surface electrode  5   b  and a positive charge is generated in the other portion of the electrode  5   b  with the border B 2  delineating the one portion from the other portion. A positive charge is generated in the one bottom major surface electrode  6   a , and a negative charge is generated in one portion of the other bottom major surface electrode  6   b  and a positive charge is generated in the other portion of the electrode  6   b  with the border B 2  delineating the one portion from the other portion. A negative charge is generated in one portion of the interlayer electrode  3   a , and a positive charge is generated in the other portion of the interlayer electrode  3   a  with the border B 1  delineating the one portion from the other portion. Generated charges in the electrodes  5   b ,  6   b , and  3   a  cancel each other. A negative charge is generated in the interlayer electrode  3   b.    
     As a result, the positive charge is picked up through the external electrode  7  connected to the electrodes  5   a  and  6   a , and the negative charge is picked up through the external electrode  8  connected to the interlayer electrode  3   b.    
     The piezoelectric element  2  is preferably manufactured in the following manner. 
     Two layers of piezoelectric ceramic as green sheets are prepared. Electrode paste for defining the interlayer electrodes  3   a  and  3   b  is printed on one green sheet, and then, the other green sheet is compression bonded onto the one green sheet. The laminate is baked. Electrode paste is then printed to define the major surface electrodes  5   a ,  5   b ,  6   a , and  6   b , and is then subjected to baking. Predetermined voltages are applied between the major surface electrodes and between each of the major surface electrodes and the interlayer electrode for the polarization process. To use the interlayer electrodes  3   a  and  3   b  in the polarization process, the interlayer electrode  3   a  may be outwardly routed in a direction that is substantially perpendicular to the page of FIG. 2 although no such routing is shown in FIG.  2 . By cutting a two-layered ceramic plate into substrates having a predetermined size, the piezoelectric element  2  is obtained. 
     In another manufacturing method, the interlayer electrodes and the major surface electrodes are printed on two already baked ceramic plates. The ceramic plates are then baked, and then bonded together using an adhesive agent. 
     Table 2 lists electrical characteristics of the independent cells ( 1 ) through ( 6 ) when acceleration is applied thereto. 
     The cells have the same voltage sensitivity V generated therewithin, and the area of each of the central cells ( 2 ) and ( 5 ) is almost twice as large as that of the side cells ( 1 ), ( 3 ), ( 4 ), and ( 6 ). The cells ( 2 ) and ( 5 ) thus have a charge sensitivity Q that is about twice as large as that of the side cells ( 1 ), ( 3 ), ( 4 ), and ( 6 ), and capacitance C that is about twice as large as that of the side cells ( 1 ), ( 3 ), ( 4 ), and ( 6 ). Since energy is E=VQ/2, the energy of the cells ( 2 ) and ( 5 ) is about twice the energy of the other cells. 
     
       
         
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                 Characteristics 
                 Cells (1), (3), (4), and (6) 
                 Cells (2) and (5) 
               
               
                   
               
             
             
               
                 Voltage sensitivity 
                 1 V 
                 1 V 
               
               
                 Charge sensitivity 
                 1 Q 
                 2 Q 
               
               
                 Capacitance 
                 1 C 
                 2 C 
               
               
                 Energy 
                 1 E 
                 2 E 
               
               
                   
               
             
          
         
       
     
     Table 3 lists, for comparison, the voltage sensitivity, charge sensitivity, capacitance, and energy of the conventional acceleration sensors shown in FIGS. 8A and 9A, the acceleration sensor  1 A of the first preferred embodiment of the present invention, and an acceleration sensor  1 B of a second preferred embodiment to be discussed later. 
     
       
         
               
               
               
               
               
             
           
               
                 TABLE 3 
               
               
                   
               
               
                   
                 Conventional 
                 Conventional 
                   
                   
               
               
                   
                 sensor 
                 sensor 
                 First 
                 Second 
               
               
                 Characteristics 
                 (FIG. 8A) 
                 (FIG. 9A) 
                 embodiment 
                 embodiment 
               
               
                   
               
             
             
               
                 Voltage 
                 2 V 
                 1 V 
                   3 V 
                   6 V 
               
               
                 sensitivity 
               
               
                 Charge 
                 4 Q 
                 8 Q 
                 2.4 Q 
                 1.2 Q 
               
               
                 sensitivity 
               
               
                 Capacitance 
                 2 C 
                 8 C 
                 0.8 C 
                 0.2 C 
               
               
                 Energy 
                 4 E 
                 4 E 
                 3.6 E 
                 3.6 E 
               
               
                   
               
             
          
         
       
     
     As understood from Table 3, the three cells ( 1 ), ( 2 ), and ( 3 ) are serially connected in the piezoelectric layer  2   a , and the three cells ( 4 ), ( 5 ), and ( 6 ) are serially connected in the piezoelectric layer  2   b  in the acceleration sensor  1 A. The voltages in the cells are thus summed, and the generated voltage in each layer is increased. The voltage sensitivity is thus increased to be about 1.5 times to about 3 times as high as that in the parallel connection type (see FIGS.  8 A and  9 A). In connection with the energy, the first preferred embodiment obtains about 3.6 E while the conventional sensors obtain about 4E. The difference therebetween is not significant. The acceleration sensor  1 A of the first preferred embodiment increases the voltage sensitivity in the sensitivity characteristics while controlling a decrease in the generated energy. The present invention thus provides an acceleration sensor which is appropriate for use in an operating environment where internal noise is high in level. 
     The major surface electrodes and the interlayer electrodes face each other with the piezoelectric layers  2   a  and  2   b  interposed therebetween. The thickness of each of the piezoelectric layers  2   a  and  2   b  is preferably as thin as about 100 μm. The characteristics that take into consideration the effect of the shape including the thickness are expressed as follows: 
     
       
         Charge sensitivity  Q=kd·WL   3   /T   
       
     
     
       
         Voltage sensitivity  V=kg·L   2   
       
     
     where L is the length between supported points of the support frames, W is the width of the detector element, T is the thickness of the detector element, g and d are piezoelectric constants, and k is a coefficient. 
     As understood from the above equations, the thinner the piezoelectric element, the higher the charge sensitivity. Since the voltage sensitivity is not related to the thickness of the piezoelectric element, the thinner the element, the higher the energy becomes. A thinner design of the element contributes to the manufacturing of a high-sensitivity sensor. On the other hand, insulation must be provided by maintaining a gap, corresponding to the thickness of the piezoelectric layer, between the major surface electrode and the interlayer electrode on the side of the piezoelectric element. If the thickness of the piezoelectric layer is too small, sufficient resistance between the electrodes is not assured. Reduction in insulation between the electrodes does not occur uniformly on the entire end surface of the element but in a localized position where impurity that is mixed during a process typically reacts with moisture ingressing into a package surrounding the detector element. 
     In this preferred embodiment, the three cells are connected in series in each layer. Even if a particular cell is degraded in insulation, the remaining two maintain insulation. The degree of degradation is thus reduced. Even if one single cell is completely shorted, two-thirds of the overall insulation of the cells is still assured. As the insulation requirement is reduced, the thickness of the piezoelectric layer is reduced and sensitivity is increased. 
     In the conventional sensors (shown in FIGS. 8A and 8B and FIGS.  9 A and  9 B), the polarization direction must be inverted at the border between the three regions arranged in the longitudinal direction. During the polarization process, an external electrode is split into three regions, voltages corresponding to the polarization directions are applied, and the external electrodes are then connected. Since the polarization directions in the length of the element are the same in this preferred embodiment, the top and bottom major surface electrodes are formed in the final form thereof, and then the polarization is achieved using the major surface electrodes. The ease of production of the sensor is thus increased, and the manufacturing costs of the sensor are reduced. 
     FIGS. 5 through 7 illustrate an acceleration sensor  1 B in accordance with a second preferred embodiment of the present invention. 
     In the acceleration sensor  1 B of the second preferred embodiment, the polarization directions of the two piezoelectric layers  2   a  and  2   b  defining the piezoelectric element  2  are inverted. Three separate interlayer electrodes are used. Bottom major surface electrodes  6   a  and  6   b  are point-symmetrical with top major surface electrodes  5   a  and  5   b . Components that are identical to those in the first preferred embodiment are designated with the same reference numerals, and the discussion thereof is omitted here. 
     The interlayer electrodes  3   a ,  3   b , and  3   c  arranged in the longitudinal direction are separated along borders B 1  and B 2 . None of the interlayer electrodes  3   a ,  3   b , and  3   c  is routed to the longitudinal end surfaces of the piezoelectric element  2 , and none of the interlayer electrodes  3   a ,  3   b , and  3   c  is connected to the external electrodes  7  and  8 . 
     The major surface electrodes  5   a  and  5   b  arranged on the major surface in the longitudinal direction are separated along the border B 1 . The electrode  5   a  extends with one end thereof routed to the left end surface of the piezoelectric element  2  and the other end terminated near the border B 1 . The electrode  5   b  extends with one end thereof terminated near the border B 1  and the other end thereof terminated near the right hand end surface of the piezoelectric element  2 . 
     The bottom major surface electrodes  6   a  and  6   b  arranged in the longitudinal direction are separated along the border B 2 . The major surface electrode  6   a  extends with one end thereof terminated near the left hand end surface of the piezoelectric element  2  and the other end terminated near the border B 2 . The major surface electrode  6   b  extends with one end thereof terminated near the border B 2  and the other end thereof routed to the right hand end surface of the piezoelectric element  2 . To pick up the charge generated in the piezoelectric element  2 , the top major surface electrode  5   a  is connected to the external electrode  7 , and the bottom major surface electrode  6   b  is connected to the external electrode  8 . 
     The six cells ( 1 ) through ( 6 ) are connected in series as shown in FIG.  6 . 
     When acceleration G acts on the acceleration sensor  1 B, a tensile stress occurs in the central cell ( 2 ) in the top piezoelectric layer  2   a , and a compressive stress occurs in the side cells ( 1 ) and ( 3 ). A compressive stress occurs in the central cell ( 5 ) in the bottom piezoelectric layer  2   b , while a tensile stress occurs in the side cells ( 4 ) and ( 6 ). Depending on the relationship between the above-mentioned stress and the polarization direction P, a positive charge is generated in the one top major surface electrode  5   a  and a negative charge is generated in one portion of the other top major surface electrode  5   b  and a positive charge is generated in the other portion of the electrode  5   b  with the border B 2  delineating the one portion from the other portion. A negative charge is generated in one portion of the one bottom major surface electrode  6   a  and a positive charge is generated in the other portion of the electrode  6   a  with the border B 1  delineating the one portion from the other portion, and a negative charge is generated in the other bottom major surface electrode  6   b . A positive charge and a negative charge generated in each of the electrodes  3   a ,  3   b , and  3   c  cancel each other. 
     As a result, the positive charge is picked up through the external electrode  7  connected to the electrodes  5   a  and  6   a , and the negative charge is picked up through the external electrode  8  connected to the electrode  6   b.    
     All cells ( 1 ) through ( 6 ) are serially connected in the acceleration sensor  1 B of the second preferred embodiment. As listed in Table 3, the voltage sensitivity of the second preferred embodiment is about 6 V, which is higher than not only the conventional sensors but also the sensor of the first preferred embodiment. In connection with the energy, the second preferred embodiment achieves about 3.6E while the conventional sensors achieve about 4E. The difference therebetween is not significant. The acceleration sensor  1 B of the second preferred embodiment increases the voltage sensitivity in the sensitivity characteristics while controlling a decrease in the generated energy. 
     As in the first preferred embodiment, a cell having a degraded insulation does not affect the other cells in the second preferred embodiment. The performance degradation is thus controlled. Specifically, since the effect of reduced insulation is alleviated, thinner piezoelectric layers  2   a  and  2   b  work. A high-sensitivity acceleration sensor is thus provided. 
     Since the three cells in each of the layers  2   a  and  2   b  have the same polarization directions, the polarization is easy to achieve. 
     The present invention is not limited to the above-described preferred embodiments. 
     The polarization directions P of the piezoelectric layers  2   a  and  2   b  face away from each other in the acceleration sensor  1 B as shown in FIG.  5 . If the polarization directions P of the piezoelectric layers  2   a  and  2   b  are set to face each other, the present invention operates in the same way, and provides the same advantages. But the polarity of the generated charge becomes opposite. 
     The acceleration sensors  1 A and  1 B may be formed of three or more layers. 
     It should be understood that the foregoing description is only illustrative of the present invention. Various alternatives and modifications can be devised by those skilled in the art without departing from the present invention. Accordingly, the present invention is intended to embrace all such alternatives, modifications and variances which fall within the scope of the appended claims.
An acceleration sensor includes a piezoelectric element and a support member for supporting the piezoelectric element at both longitudinal ends thereof. The piezoelectric element includes a laminate of at least two piezoelectric layers. Each of the at least two piezoelectric layers includes three longitudinally aligned regions separated at two borders where stress is inverted in the longitudinal direction of the piezoelectric element when acceleration is applied. Cells, each formed of a respective region, are polarized in the same direction of thickness in each of the two external piezoelectric layers. Electrodes are arranged so that the three cells in the one piezoelectric layer are serially connected, the three cells in the other piezoelectric layer are serially connected, and then the three serially connected cells in the one piezoelectric layer and the three serially connected cells in the other piezoelectric layer are connected in parallel. Electrodes are routed out to the different longitudinal ends of the piezoelectric element.