Abstract:
A stress sensitive element includes a vibrating arm having an electrode, a beam portion integrated with the vibrating arm at both ends of the vibration arm, and a connecting portion interposed between the beam portion and the vibrating arm. The element is placed by providing a fixing portion to one of the connecting portion and the beam portion, and an extending direction of the vibrating arm is orthogonal to a stress direction to be detected.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Technical Field 
         [0002]    The present invention relates to a stress sensitive element, in particular, to one suitably used for detection of stress such as acceleration by utilizing change in resonance frequency of a piezoelectric resonator. 
         [0003]    2. Related Art 
         [0004]    Acceleration sensors have been widely used not only in automobiles, airplanes and rockets but also in various industrial plants for the use of, for example, monitoring abnormal vibration. JP-A-2-248866 discloses a beam structure of an acceleration sensor shown in a perspective view of  FIG. 6 . In  FIG. 6 , the reference symbol  50  denotes a double-ended tuning fork type quartz resonator having two vibrating beams  51 , the reference symbol  52  denotes a bonding portion of the double-ended tuning fork type quartz resonator  50 , and the reference symbol  60  denotes a beam constituted by quartz having the same cutting angle as that of the double-ended tuning fork type quartz resonator  50 . The beam  60  has projecting portions  70  having a thicker thickness than that of other parts thereof. The projecting portions  70  and the bonding portion  52  of the double-ended tuning fork type quartz resonator  50  are bonded by using adhesive or the like. Then, a weight  80  is provided on a free end of the beam  60  and the other end opposite to the weight  80  is fixed to a base  90 . 
         [0005]    The base  90  of the acceleration sensor constructed as shown in  FIG. 6  is fixed on an object to be measured. When the acceleration is applied in the direction of the arrow, the weight  80  makes the beam  60  bend, and compression stress or extension stress is then applied to the double-ended tuning fork type quartz resonator  50  fixed to the beam  60 , resulting in change of the frequency. In sum, the acceleration sensor is one that determines the magnitude of acceleration from the amount of the frequency change. According to the disclosure of JP-A-2-248866, the projecting portions  70  are formed on the beam  60 , so the magnitude of stress applied to the double-ended tuning fork type quartz resonator  50  is increased as compared with the case in which no projecting portions, thereby making it possible to provide an acceleration sensor of high sensitivity while preventing the plate thickness of the beam  60  from becoming thinner and avoiding increase in the mass of the weight  80 . 
         [0006]    However, the acceleration sensor disclosed in JP-A-2-248866 is constructed in a way that the beam  60  constructed by quartz having the same cutting angle as that of the double-ended tuning fork type quartz resonator  50  is bonded to the resonator and the weight is bonded to the free end of the beam  60 . As a result, there arises a problem in that the number of man-hours increases and the manufacturing cost becomes high. 
       SUMMARY 
       [0007]    An advantage of the present invention is to provide a stress sensitive element capable of steadily detecting stress with low cost. 
         [0008]    A stress sensitive element according to one aspect of the invention includes: a vibrating arm having an electrode; a beam portion integrated with the vibrating arm at both ends of the vibration arm; and a connecting portion interposed between the beam portion and the vibrating arm. The element is placed by providing a fixing portion to the connecting portion or the beam portion, and an extending direction of the vibrating arm is orthogonal to a stress direction to be detected. As thus constructed, the fabrication can be easily performed by, for example, etching process, so the manufacturing cost can be reduced. 
         [0009]    In this case, the fixing portion may be provided to the connecting portion, and a constriction provided to the beam portion. As thus constructed, it is possible to allow the beam portion to easily bend due to the constriction, so the stress in a single direction can be sensitively detected. 
         [0010]    In this case, the central portion of the beam portion may be used as a fixing portion. As thus constructed, the stress in a single direction can be sensitively detected. 
         [0011]    In this case, the element may include a plurality of constrictions provided to the beam portion in one or more symmetrical pairs on either side of the fixing portion in the extending direction of the vibrating arm. As thus constructed, it is possible to allow the beam portion to easily bend due to the constriction, so the stress in a single direction can be sensitively detected. 
         [0012]    It is preferable that the vibrating arm be a double-ended tuning fork type. 
         [0013]    As thus constructed, the properties of stress—frequency change can be enhanced, so the stress detection sensitivity can be enhanced and the degree of stability can be increased. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The invention will be described with reference to the accompanying drawings, wherein like numbers reference like elements. 
           [0015]      FIG. 1A  is a schematic perspective view illustrating the structure of a stress sensitive element according to an embodiment of the invention. 
           [0016]      FIG. 1B  is a sectional view showing electrodes and the connection thereof. 
           [0017]      FIG. 2  is a perspective view illustrating the structure of a stress sensitive element according to a second embodiment of the invention. 
           [0018]      FIG. 3  is a perspective view illustrating the structure of a stress sensitive element according to a third embodiment of the invention. 
           [0019]      FIG. 4  is a perspective view illustrating the structure of a stress sensitive element according to a fourth embodiment of the invention. 
           [0020]      FIG. 5  is a plan view illustrating the structure of a stress sensitive element according to a fifth embodiment of the invention. 
           [0021]      FIG. 6  is a perspective view showing a related art acceleration sensor. 
       
    
    
     DESCRIPTION OF EXEMPLARY EMBODIMENTS 
       [0022]    Hereinafter, embodiments of the invention will be described with reference to the drawings. 
         [0023]      FIG. 1A  is a perspective view illustrating the structure of a stress sensitive element  1  according to an embodiment of the invention. 
         [0024]    The stress sensitive element  1  shown in  FIG. 1A  is made of quartz, which is a piezoelectric material. The stress sensitive element  1  has a vibrating arm  20  and beam portions  10   b  each integrally formed in the both ends of the vibrating arm  20 . That is, an end of the connecting portion  10  and an end of the beam portion  10   b  are bonded to each other, the other end of the beam portion  10   b  and an end of the connecting portion  10   c  are bonded to each other, and the both ends of the vibrating arm  20  are bonded to the other end of the connecting portion  10   a  and the other end of the connecting portion  10   c.  The stress sensitive element  1  thus constructed can be integrally formed by performing etching process to a quartz plate by utilizing known photolithography technique and etching means. 
         [0025]    The vibrating arm  20  has first, second, third electrode portions  22 ,  23 ,  24  adjacent to each other, a lead electrode (not shown), and terminal electrodes  25 ,  26  provided on the respective ends of the vibrating arm  20 . The first, second and third electrode portions  22 ,  23 ,  24  each have an electrode on four planes except the both ends of the vibrating arm  20  and connect the respective opposite electrodes of the respective electrode portions by the lead electrode. Further, an upper surface electrode of the first electrode portion  22  (XZ plane), a lateral surface electrode of the second electrode portion  23  (XY plane) and an upper surface electrode of the third electrode portion  24  (XZ plane) are connected to a terminal electrode  25  formed on the end of the vibrating arm  20  by using the lead electrode. Furthermore, a lateral surface electrode of the third electrode portion  24  (XY plane), an upper surface electrode of the second electrode portion  23  (XZ plane) and a lateral surface electrode of the third electrode portion  24  (XY plane) are connected to a terminal electrode  26  formed on the end of the vibrating arm  20  by using the lead electrode. 
         [0026]      FIG. 1B  is a sectional view showing the electric charge generated at a moment on electrodes  23   a,    23   b,    23   c  and  23   d  of the second electrode portion  23  and the connection of the electrodes  23   a,    23   b,    23   c  and  23   d.    
         [0027]    The first, second, third electrode portions  22 ,  23 ,  24  are formed on the vibrating arm  20 . When a high frequency voltage is applied to the terminal electrodes  25  and  26 , the vibrating arm  20  performs a flexural vibration with a natural frequency within the X-Z plane while its both ends serve a node. The natural frequency (resonance frequency) of the vibrating arm  20  changes depending on stress (compression or extension) applied to the vibrating arm  20 . 
         [0028]    As shown in  FIG. 1A , a central portion of the connecting portion  10   c  adjacent to the vibrating arm  20  is fixed (as a fixing portion P) and the Z-axis direction, orthogonal to an extending direction of the vibrating arm  20 , is defined as a direction of an acceleration detection axis. For example, the acceleration occurs in the +Z-axis direction (direction of arrow), the stress due to inertia force in the minus Z-axis direction is applied to the stress sensitive element  1 , and the beam portion  10   b  is bent (warped) in this direction since the point P is fixed. At this time, such an effect is caused as to narrow the distance between the connecting portion  10   a  and the connecting portion  10   c,  so the compression stress is applied to the vibrating arm  20  and the resonance frequency changes so as to become lower. On the other hand, when the acceleration occurs in the minus Z-axis direction (opposite direction to the direction of arrow), the stress due to inertia force in the +Z-axis direction is applied to the stress sensitive element  1 , and the beam portion  10   b  of the stress sensitive element  1  is bent (warped) in this direction. At this time, such an effect is caused as to broaden the distance between the connecting portion  10   a  and the connecting portion  10   c,  so the extension stress is applied to the vibrating arm  20  and its resonance frequency changes so as to be higher. It is possible to determine the applied stress by detecting this frequency change. 
         [0029]    In the case of the stress sensitive element  1  according to this embodiment of the invention, a flat quartz plate is widely etched in its central portion by utilizing a photolithography technique and etching means so that only the peripheral portion thereof remains, thereby making a quartz structure having a rectangular shape. At one end of this quartz structure, there are formed, by using deposition means or the like, the first, second and third electrode portions  22 ,  23 ,  24 , and the lead electrode (not shown) and the terminal electrodes  25 ,  26 , as shown in  FIG. 1A . 
         [0030]    As thus constructed, the thermal expansion coefficient of the structure becomes uniform, thereby making it possible to advantageously minimize the influence of the temperature change. Furthermore, the quartz plate is processed by utilizing a photolithography technique and etching means, thereby also making it possible to advantageously form the functional part freely, enhance the accuracy of finishing and achieve miniaturization while only a few man-hours are needed. 
         [0031]      FIG. 2  is a perspective view illustrating the structure of a stress sensitive element  2  according to a second embodiment of the invention. 
         [0032]    The stress sensitive element  2  is different from the stress sensitive element  1  shown in  FIG. 1A  in that a rounded constriction  11  is formed in the central portion of the beam portion  10   b  on the opposite end to the vibrating arm  20 . 
         [0033]    As shown in  FIG. 2 , since the constriction  11  is formed in the beam portion  10   b,  so the bending portion  10   b  can easily bend upon the stress applied to the axis direction for acceleration detection (Z-axis direction), so the stress detection sensitivity is advantageously enhanced. 
         [0034]      FIG. 3  is a perspective view illustrating the structure of a stress sensitive element  3  according to a third embodiment of the invention, which is different from the stress sensitive element  1  shown in  FIG. 1A  in that the portion, at which the stress sensitive element  3  is fixed, is made substantially the center portion of the beam portion  10   b  on the opposite side to the vibrating arm  20 . As shown in  FIG. 3 , since the substantially central portion of the beam portion  10   b  is fixed, so when the acceleration occurs in the axis direction for acceleration direction (Z-axis direction), the beam portion  10   b  bends while the fixing portion P located at the central portion becomes on the top or on the bottom thereof. 
         [0035]      FIG. 4  is a perspective view illustrating the structure of a stress sensitive element  4  according to a fourth embodiment, which is different from the stress sensitive element  1  shown in  FIG. 1A  in that the substantially central portion of the beam portion  10   b  on the opposite side to the vibrating arm  20  is made as a fixing portion P and, in addition to this, constrictions  12 ,  13  are provided across the point P. By thus constructing as shown in  FIG. 4 , the beam portion  10   b  easily bends upon the stress applied in the axis direction for acceleration detection (Z-axis direction), so the stress detection sensitivity is advantageously enhanced. 
         [0036]      FIG. 5  is a plan view illustrating the structure of a stress sensitive element  6  according to a fifth embodiment of the invention, which is different from the stress sensitive element  1  shown in  FIG. 1  in that a double-ended tuning fork type quartz resonator element  27  is used as the vibrating arm. The relationship between the change of the resonance frequency of the double-ended tuning fork type quartz resonator element  27  and the stress (compression or extension) applied is represented by a substantially straight line, as is commonly known. 
         [0037]    Therefore, provided that the resonance frequency of the double-ended tuning fork type quartz resonator element  27  is made to be f 0  in the case in which no stress is applied to the double-ended tuning fork type quartz resonator element  27 , as the extension stress is applied to the double-ended tuning fork type quartz resonator element  27 , the resonance frequency becomes higher than f 0 , and as the compression stress is applied, the resonance frequency becomes lower than f 0 . As a result, the relationship between the applied stress and the resonance frequency is represented by the substantially straight line. 
         [0038]    The fixing portion P for the stress sensitive element  6  shown in  FIG. 5  may be in the substantially central portion of the connecting portion  10   c  or in the central portion of the beam portion  10   b.  Further, a constriction may be provided in the substantially central portion of the beam portion  10   b,  as shown in  FIG. 2  and  FIG. 4 , or on the either side thereof sandwiching the central portion. 
         [0039]    It should be noted that the above mentioned embodiments of the invention have been explained while taking the cases as examples in which flat quartz is processed by using a photolithography technique and etching means so that a rectangular shaped quartz structure is formed. However, it should not be construed as restrictively. It is also possible to form a functional member having a reversed C shape by bonding the ends of the connecting portions  10   a,    10   c  and the beam portion  10   b  made of an elastic material having elasticity and to form a rectangular-shaped stress sensitive element by bonding the both ends of the vibrating arm  20  to the functional member. Moreover, it is also possible to perform etching process to a flat elastic member so as to form a functional member having a reversed C shape and to constitute a rectangular-shaped stress sensitive element by bonding the both sides of the vibrating arm  20  to the openings thereof. As thus constructed, the accuracy of finishing is enhanced and the cost can be reduced as compared to the case in which the ends of the connecting portions  10   a,    10   c  and the beam portion  10   b  are bonded to each other, thereby forming a functional member having a reversed C shape. Furthermore, in the case in which the functional member is formed by performing etching process, the shape of the functional member can be more freely selected. For example, the U-shaped functional member can be formed by etching and the vibrating arm is bonded to the opening thereof, thereby constituting a stress sensitive element. 
         [0040]    Yet further, the stress sensitive element  1  shown in  FIG. 1A  is constructed in a way that the central portion P of the connecting portion  10   c  is fixed. As a result, there may be a case in which inertia force is generated on the connecting portion  10   a  upon the acceleration in the X-axis direction (the extending direction of the vibrating arm  20 ), so the effect of compression or extension of the vibrating arm  20  can be likely to occur. 
         [0041]    With respect to this, as in the case with the stress sensitive element  3  shown in  FIG. 3 , for example, in which the portion, at which the stress sensitive element  3  is fixed, is in the substantially central portion of the beam portion  10   b  on the opposite side to the vibrating arm  20 , the connecting portions  10   a  and  10   c  can bend in the same direction upon the acceleration in the X-axis direction. Therefore, the distance between the connecting portions  10   a  and  10   c  can be easily kept the same both prior to and subsequent to the acceleration, so there is an advantage that the effect causing the compression or extension of the vibrating arm  20  hardly occurs.