Acceleration sensing device

An acceleration sensing device includes: an outer frame; a first drive arm having both ends supported by sides of the outer frame through respective base parts, the sides opposing each other; a second drive arm extending from one of the base parts of at least one of the sides toward the other side; and a sensing arm that is disposed midway between the first drive arm and the second drive arm and extends form the one base part of the one side toward the other side, the sensing arm having an electrode in order to extract electric charge generated in the sensing arm. In the device, the first drive arm and the second drive arm have excitation electrodes for a flexural vibration and form a tuning fork type resonator, and center positions in thicknesses of sections that are located in the base parts and on an extension line of the first drive arm differ from a center position in a thickness of the first drive arm.

BACKGROUND

1. Technical Field

The present invention relates to an acceleration sensing device, particularly to an acceleration sensing device in which sensitivity in an acceleration detection axis direction is improved and sensitivities in other axis directions are suppressed.

2. Related Art

Acceleration sensors are widely used in various things ranging from automobiles, aircrafts, and rockets to abnormal vibration monitoring units installed in plants and the like. As an acceleration sensor for household appliances, a Micro Electro Mechanical System (MEMS) sensor whose acceleration sensing structure is fabricated by using a semiconductor process technique has been commonly known.

JP-A-2006-64397 is an example of related art. The example discloses an acceleration sensing element.FIG. 5Ais a plan view of a known acceleration sensing element81. The acceleration sensing element81has a base part82, a pair of drive vibrating parts83,84that protrude out from edges of the base part82in parallel each other, and a sensing vibrating part85. The drive vibrating parts83,84and the sensing vibrating part85are long and narrow flexural vibrating arms. The drive vibrating part83has a pair of grooves86a,86b(not shown in the drawing) on its upper and lower faces respectively. An electrode88is provided inside and on the sidewall of the grooves86a,86b. The electrode88is an electrode for exciting the drive vibrating part83in such a way indicated by the arrow “A” shown inFIG. 5B. The drive vibrating part83includes a strip-shaped main portion and a wide portion96at its end part.

The other drive vibrating part84has a strip and plate like shape. An electrode87is disposed on the surface of the drive vibrating part84. The electrode87is an electrode for exciting the drive vibrating part84in such a way indicated by the arrow “A” shown inFIG. 5B. The strip-and-plate shaped sensing vibrating part85has a pair of grooves89a,89b(not shown in the drawing) on its upper and lower faces respectively. An electrode90is provided in the grooves89a,89band on a side wall of the vibration part. The electrode90is an electrode through which it is possible to detect vibration of the sensing vibrating part85in the direction indicated by the arrow “C” in the drawing.

Referring toFIG. 5B, the drive vibrating parts83,84in the acceleration sensing element81vibrate flexuously and in the phase opposite each other as indicated by the arrow “A”. Moreover, vibration frequencies of the drive vibrating parts83,84at the time of self-excited oscillation are made the same, which means that an amplitude of the sensing vibrating part85disposed at the center becomes zero when no acceleration is applied.

When acceleration is applied to the acceleration sensing element81in the direction indicated by the arrow “B” inFIG. 5B, a force is applied to the drive vibrating parts83,84in an X axis direction. The drive vibrating parts83,84thereby extend in the X axis direction and their vibration frequencies both increase. The frequency change (increase) of the drive vibrating part83is larger than the frequency change (increase) of the drive vibrating part84since the drive vibrating part83has the wide portion96that holds a large weight at its end. Consequently moments “mα” (m: mass, α: acceleration) of the drive vibrating parts differ each other, and which generates a flexural vibration of the sensing vibrating part85in a Y axis direction as denoted by the arrow “C” shown inFIG. 5B. The example describes that an amplitude of the flexural vibration “C” monotonically increases as the acceleration “B” increases and the amplitude is substantially proportional to an output from a detection electrode of the sensing vibrating part85thereby it is possible to obtain the acceleration “B”.

However the acceleration sensing element disclosed in the example made one of the drive vibrating parts have a larger weight compared to that of the other vibration part in order to unbalance the vibration system. For this reason, it is difficult to realize both a small sized sensing element and to improve the sensitivity.

Moreover the acceleration sensing element disclosed by the example has a disadvantage of sensitivity in an unintended axis. The sensing element of the example has more than one sensible acceleration axis (the X axis direction inFIG. 7) with which the vibration system is unbalanced, thereby acceleration in other direction is also detected when the acceleration is applied to for example the orthogonal direction to the drive vibrating part.

SUMMARY

An advantage of the present invention is to provide a small-sized and highly sensitive acceleration sensing element in which sensitivities of other axes are suppressed.

An acceleration sensing device according to the invention includes: an outer frame; a first drive arm having both ends supported by sides of the outer frame through respective base parts, the sides opposing each other; a second drive arm extending from one of the base parts of at least one of the sides toward the other side; and a sensing arm that is disposed midway between the first drive arm and the second drive arm and extends form the one base part of the one side toward the other side, the sensing arm having an electrode in order to extract electric charge generated in the sensing arm. In the device, the first drive arm and the second drive arm have excitation electrodes for a flexural vibration and form a tuning fork type resonator, and center positions in thicknesses of sections that are located in the base parts and on an extension line of the first drive arm differ from a center position in a thickness of the first drive arm.

According to the invention, the first and second arms are provided and the sensing arm is disposed therebetween, the first and second drive arms are excited to oscillate at the same frequency but in an opposite phase. In this way, no vibration is excited in the sensing arm when no acceleration is applied but a flexural vibration is generated in the sensing arm when acceleration is applied. Consequently electric charge is excited and the value of the applied acceleration can be obtained. Moreover there is another advantage is that the sensitivity detecting acceleration is high because the outer frame is provided and sensitivities in other axes are suppressed.

In this case, both ends of the second drive arm may be supported by the opposing sides of the outer frame through the respective base parts, and center position in thicknesses of sections that are located in the base parts and on an extension line of the second drive arm differ from a center position in a thickness of the second drive arm; and the center positions in the thicknesses of the sections of the first drive arm may differ from the center positions in the thicknesses of the sections of the second drive arm.

When the center positions in the thicknesses of sections of the base parts located on the extension lines of the first and second drive arms differ from the center positions of the drive arms, there is no oscillation is excited in the sensing arm at the time of no acceleration application. But when acceleration is applied, the oscillating system is imbalanced, oscillation is excited in the sensing arm, and it becomes possible to obtain the value of the acceleration from the amount of electric charge. Since the oscillation of the arms is balanced by setting it at an opposite phase, it is possible to increase the detection sensitivity of the acceleration sensing device when acceleration is applied.

In the acceleration sensing device, the outer frame may have a first narrowed section and a second narrowed section, the second narrowed section being disposed at a position remote from the first narrowed section with an opening of the outer frame interposed between the first and the second narrowed sections and in a direction orthogonal to an extended direction of the first drive arm.

By providing the first and second narrowed sections in this way, it is possible to enhance the sensitivity to detect acceleration.

In the acceleration sensing device, the base parts may have a projecting shape being provided to inner edge parts of the opposing sides, both ends of the first and the second drive arms being integrally formed with the respective base part, a base edge part of the sensing arm being integrally formed with the one base part, a first concave section and a second concave section being provided on one face of the base parts, and a third concave section and a fourth concave section being provided on other face of the base parts.

As described above, the first and second drive arms are provided between the base parts of the outer frame. The sensing arm is held at the base part of the one side. The concave sections are provided in the base edge parts and on one face of the first drive arm. The other concave sections are provided in the base edge parts and on the other face of the second drive arm. These concave sections are disposed in the point-symmetrical manner, and the first and second drive arms are excited to oscillate in an opposite phase each other. Thereby oscillation is not generated when no acceleration is applied. When acceleration is applied, the resonance frequency of the first drive arm changes in an opposite way to that of the second drive arm therefore the oscillating system can be efficiently imbalanced. Moreover, it is possible to increase the detection sensitivity of acceleration.

In the acceleration sensing device, the excitation electrodes disposed in the first and second drive arms may be provided such that the first and the second drive arms are both excited in a mode of the flexural vibration but in an opposite phase each other.

In this way, the first and second drive arms are oscillated in an opposite phase each other so that oscillation is not excited in the sensing arm when no acceleration is applied. When acceleration is applied, the oscillating system is imbalanced and oscillation is excited in the sensing arm. As a result, it is possible to obtain the value of the acceleration by utilizing electric charge that is excited by the oscillation of the sensing arm. In addition, frequency of the first drive arm changes in an opposite direction to that of the third drive arm therefore it is possible to determine the direction in which the acceleration is applied.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Embodiments of the invention will be described.

First Embodiment

FIG. 1is a schematic perspective view of an acceleration sensing device1according to a first embodiment of the invention showing its structure.

The acceleration sensing device1includes a contour vibration body1awhich is a piezoelectric substrate, and an electrode1bthat is provided on the contour vibration body1a.

The contour vibration body1aincludes an outer frame5having a rectangular-frame shape, a first drive arm10, a second drive arm11, and a sensing arm12. The rectangular-frame shaped outer frame5has two longer sides5a,5band two shorter sides5c,5d.

The contour vibration body1ahas the first drive arm10whose both ends are supported by the two opposing shorter sides5c,5d(edges) of the rectangular-frame shaped outer frame5and that extends in parallel with the longer sides5a,5b(edges) of the outer frame5. The contour vibration body1aalso has the second drive arm11. The second drive arm11has a cantilever structure in which a base edge part11aof the second drive arm is fixed by a projecting base part6adisposed at an inner edge of the side5cand the arm extends toward the shorter side5dand in parallel with the longer sides5a,5b. The contour vibration body1afurther has the sensing arm12. The sensing arm12has a cantilever structure in which a base edge part12aof the sensing arm is fixed by the base part6aand the arm extends toward the shorter side5d(edge) and in parallel with the longer sides5a,5b. The contour vibration body1afurther has concave sections15a,15band narrowed sections15′c,15′d. The concave sections15a,15bare disposed at positions corresponding to the ends (the base parts6a,6b) of the first drive arm10and in the same plane with the two shorter sides5c,5drespectively. The narrowed sections15′c,15′dare provided at the opposing two (longer) sides5a,5bof the outer frame5in a direction perpendicular to the face having the two sides and at the position close to the shorter side5cso as to oppose each other. The sensing arm12is disposed at the middle between the first drive arm10and the second drive arm11.

In other words, the base part6a, which is integrally formed with and protrudes out from the inner edge of the shorter side5c, has the concave section15ain its one face. Base edge parts10a,11aof the first and second drive arms10,11and the base edge part12aof the sensing arm12are also integrally formed with the base part6a. The other base part6b, which is integrally provided with and protrudes out from the inner edge of the shorter side5d, has the concave section15bin a face (the same plane as where the concave section15adisposed) of the base part6b. The other base edge part10bof the first drive arm10is integrally formed with the base part6b. In other words, the two concave sections15a,15bare disposed in the same plane of the base parts6a,6band disposed symmetrically to a longitudinal direction of the outer frame5.

Cross-sectional shapes of the narrowed sections15′c,15′dare preferably set to ones that can be easily bent or warped with the fulcrum points of the narrowed sections15′c,15′d. Such shape can include for example a rectangular shape as shown in the drawing, a semicircular shape, a hyperbolic shape, a wedge shape and the like.

FIG. 2Ais a plan view of an electrode provided in the contour vibration body.FIG. 2Bis sectional views of the electrodes at various parts illustrating signs of electric charge that is generated at each electrode at some moment. Excitation electrodes20(20ato20d) to22(22ato22d) are provided in the first drive arm10, and an excitation electrode23(23ato23d) is provided in the second drive arm11. A flexural vibration with both ends fixed is excited at the first drive arm10, whereas a flexural vibration with one end fixed is excited at the second drive arm11. The sensing arm12has an electrode24(24ato24d) that picks up electric charge which is generated by an one-end fixed flexural vibration of the sensing arm12.

The excitation electrodes20(20ato20d),21(21ato21d),22(22ato22d) are sequentially disposed from the base edge part10atoward the other base edge part10bon the first drive arm. Each of the electrodes20ato22dis coupled by lead electrodes (wiring electrodes) provided on the drive arm10. The electrodes are coupled through the lead electrodes based on signs of electric charge of the first drive arm10shown inFIG. 2Bsuch that the two electrodes with the same sign, in other words, two electrodes with the same positive sign (+) or two electrodes with the same negative (−) sign, are coupled each other so as to form a two-terminal structure. When alternating-current voltage is applied to the two terminals, the both-ends fixed flexural vibration is excited.

Lead electrodes that extend from the first and second drive arms10,11couple the electrodes with the same positive sign (+) or the electrodes with the same negative (−) sign, then the lead electrodes are coupled with terminal electrodes26a,26bwhich are provided on the base part6a.

The electrode24(24ato24d) is disposed on the sensing arm12. Each of the electrodes24ato24dis coupled with a lead electrode which is provided on the sensing arm12. The electrodes are coupled through the lead electrode based on signs of electric charge of the sensing arm12shown inFIG. 2Bsuch that the two electrodes with the same sign, in other words, two electrodes with the same positive sign (+) or two electrodes with the same negative (−) sign, are coupled each other so as to form a two-terminal structure. The electrodes24a-24dare provided such that the sensing arm12is excited to oscillate in the one-end fixed flexural vibration manner and the electric charge generated by the flexural vibration are picked up by the electrodes.

A case where acceleration a in a thickness direction (Z axis direction) is applied to the acceleration sensing device1as shown inFIG. 1will be now described. Before the acceleration α is applied, the lead electrodes that extend from the excitation electrodes of the first and second drive arm10,11are coupled to unshown oscillation circuits respectively and oscillate (self-excited oscillation) at the same frequency f0. The electrodes20ato20dof the first drive arm10and the electrodes23ato23dof the second drive arm11are wired such that an opposite voltage is applied to the corresponding electrodes between the first drive arm and the second drive arm. Thereby a part of the first drive arm10close to the base part6aoscillates in a flexural vibration manner with an opposite phase to the flexural vibration of the second drive arm11. More specifically, when the part of the first drive arm10adjacent to the base part6aswings to +X axis direction, the second drive arm11swings to −X axis direction. These arms oscillate at the same frequency but in the opposite phase so that the distortion in the base part6acaused by the oscillation distributes symmetrically with respect to a center line extending from the center of the sensing arm12. Therefore oscillation is balanced in the oscillating system, in other words, in the area including the first and second drive arms10,11, the sensing arm12and the base part6a, so that no vibration is excited at the sensing arm12.

When acceleration α is applied in an acceleration sensing axis direction (the +Z axis direction inFIG. 1), the outer frame5is bent (inflected) in −Z axis direction with the narrowed sections15′c,15′dwhich serves as a supporting point and with a free end5dof the acceleration sensing device1which serves as a weight part. Because the concave sections15a,15bare provided only on one face (the upper face inFIG. 1) of the base parts6a,6bat the both ends of the first drive arm10, compressive stress works in the first drive arm10that is disposed between the concave sections15a,15bwhen inertial force generated by the acceleration α works in the −Z axis direction. As a result, a resonance frequency of the first drive arm10decreases. On the contrary, when the acceleration α is applied in the −Z axis direction, in other words, when inertial force works in the +Z axis direction, extensional stress (tensile stress) works in the first drive arm10disposed between the concave sections15a,15band the resonance frequency of the first drive arm10increases.

On the other hand, the second drive arm11is not affected by flexure (bending) of the outer frame5caused by the acceleration α therefore the frequency of the second drive arm11remains unchanged. Consequently the distortion distribution in the first drive arm10and the distortion distribution in the second drive arm11caused by the oscillation becomes asymmetrical with respect to the center line. In other words, the oscillating system is imbalanced, the distortion spreads to the sensing arm12, and the one-end fixed flexural vibration is excited. An amplitude of the one-end fixed flexural vibration monotonically increases depending on the magnitude of the applied acceleration whereas the amount of electric charge excited in the sensing arm12is proportional to the amplitude of the flexural vibration. By using these two relations, it is possible to obtain the magnitude of the applied acceleration from the amount of the electric charge that is picked up by the electrodes.

Since the two concave sections15a,15bare provided only on one face of the base parts6a,6brespectively, the stress that is applied to the first drive arm10placed between the concave sections15a,15bbecomes the compressive stress or the extensional stress (tensile stress) depending on the direction in which the acceleration is applied. This means that the resonance frequency of the first drive arm10differs depending on the direction of the applied acceleration. More specifically, a frequency “f0” which is the frequency when no acceleration is applied is changed to “f0+Δf” when the extensional stress is applied to the first drive arm10whereas the frequency “f0” is changed to “f0−Δf” when the compressive stress is applied. While the resonance frequency of the second drive11is not changed by the acceleration α and stays around f0. The frequency of the oscillation excited in the sensing arm12is determined by the dimensional size of the sensing arm12, and the direction of the acceleration can be detected by utilizing a phase difference from the first drive arm10.

Second Embodiment

FIG. 3is a schematic perspective view of an acceleration sensing device2according to a second embodiment of the invention showing its structure. The identical numerals are given to the same structures as those of the first embodiment in the following description.

The acceleration sensing device2includes a contour vibration body2awhich is a piezoelectric substrate, and an electrode2bthat is provided on the contour vibration body2a. The contour vibration body2aincludes the outer frame5that has a rectangular-frame shape, the first drive arm10, a third drive arm13, and the sensing arm12. The rectangular-frame shaped outer frame5consists of the two longer sides5a,5b(edges) and the two shorter sides5c,5d(edges).

The contour vibration body2ahas the first drive arm10and the third drive arm13both whose ends are supported by the two opposing shorter sides5c,5d(edges) of the rectangular-frame shaped outer frame5and that extend in parallel with the longer sides5a,5bof the outer frame5. The contour vibration body2afurther has the sensing arm12. The sensing arm12has a cantilever structure in which the base edge part12aof the sensing arm is fixed by the shorter side5cand the arm extends toward the other edge5dand in parallel with the longer sides5a,5b.

The projecting base part6awhich is provided on the inner edge of the shorter side5chas a first concave section15′aon its one face (front face side) and a third concave section15′con its other face (back face side). The base part6bthat is integrally formed with the inner edge of the shorter side5dhas a second concave section15′bon its one face (front face side) and a fourth concave section15′don its other face (back face side). The third and fourth concave sections15′c,15′dand the first and second concave sections15′a,15′bare disposed in a point-symmetrical manner respectively.

In other words, the base part6asupports the base edge parts10a,13aof the first and third drive arms10,13so as to form a single body there while it also supports the base edge part12aof the sensing arm12. On the other hand, the base part6bsupports the other base edge parts10b,13bof the first and third drive arms10,13so as to form a single body.

The first and second concave sections15′a,15′bare disposed on the front face of the base parts6a,6brespectively and in the positions corresponding to the both ends of the first drive arm10. The third and fourth concave sections15′c,15′dare disposed on the back face of the base parts6a,6brespectively and in the positions corresponding to the both ends of the third drive arm13.

The narrowed sections15′c,15′dare provided on the opposing two sides5a,5bof the outer frame5in a direction perpendicular to the face having the two sides and at the position close to the shorter side5cso as to oppose each other. The sensing arm12is disposed at the middle between the first drive arm10and the third drive arm13.

As for cross-sectional shapes of the narrowed sections15′c,15′d, they can be preferably set to a rectangular shape, a semicircular shape, a hyperbolic shape, a wedge shape and the like such that the portion can be easily bent or warped.

FIG. 4Ais a plan view of the electrode2bprovided in the contour vibration body.FIG. 2Bis sectional views of the electrodes at various parts illustrating signs of electric charge that is generated at each electrode at some moment.

Referring toFIG. 4, the excitation electrodes20(20ato20d) to22(22ato22d) are sequentially provided on the first drive arm10in the longitudinal direction, and excitation electrodes27(27ato27d) to29(29ato29d) are sequentially provided on the third drive arm13in its longitudinal direction in the contour vibration body2a. A flexural vibration with both ends fixed is excited in the first drive arm10and the third drive arm13. The sensing arm12has the electrode24(24ato24d) that picks up electric charge which is generated by the one-end fixed flexural vibration of the sensing arm12.

Connections of the electrodes provided on the first and third arms10,13and the sensing arm12will not be described here since they have been described above with reference toFIG. 2. The first drive arm10and the third drive arm13oscillate in an opposite phase each other.

A case where acceleration “α” in the thickness direction (the Z axis direction) is applied to the acceleration sensing device2will be now described. Before the acceleration α is applied, the lead electrodes that extend from the excitation electrodes of the first and third drive arm10,13are coupled to unshown oscillation circuits respectively and oscillate (self-excited oscillation) at the same frequency f0. The electrodes20to22of the first drive arm10and the electrodes27to29of the third drive arm13are wired such that a voltage with the opposite polarity is applied to the corresponding electrodes between the first drive arm and the third drive arm, thereby the first drive arm10and the third drive arm13are excited to oscillate in the opposite phase each other in the flexural vibration mode which is the same oscillation mode with a double tuning fork oscillator. These arms oscillate at the same frequency but in the opposite phase so that the distortion in the base part6acaused by the oscillation of the first and third drive arms10,13distributes symmetrically with respect to the center line that extends from the center of the sensing arm12. Therefore oscillation is balanced in the oscillating system, in other words, in the area including the first and third drive arms10,13, the sensing arm12and the base part6a, so that no vibration is excited at the sensing arm12.

When acceleration α is applied in an acceleration sensing axis direction (+Z axis direction inFIG. 3) of the acceleration sensing device2, the outer frame5is bent in the −Z axis direction with the narrowed sections15′c,15′dwhich serves as a supporting point and with a free end5dwhich serves as a weight part. Because the first and second concave sections15′a,15′bare provided on the upper face of the base parts6a,6band in the base edge parts10a,10bof the first drive arm10, compressive stress works in the first drive arm10that is disposed between the first and second concave sections15′a,15′bwhen inertial force generated by the acceleration α works in the −Z axis direction. As a result, a resonance frequency of the first drive arm10decreases. On the contrary, when the acceleration α is applied in the −Z axis direction, in other words, when inertial force works in the +Z axis direction, extensional stress (tensile stress) works in the first drive arm10disposed between the first and second concave sections15′a,15′band the resonance frequency of the first drive arm10increases.

Moreover, third and fourth concave sections15′c,15′d(not shown in the drawing) are provided on back faces of the base parts6a,6band in the base edge parts13a,13bof the third drive arm13, and extensional stress (tensile stress) works in the third drive arm10that is disposed between the third and fourth concave sections15′c,15′dwhen inertial force generated by the acceleration α works in the −Z axis direction. As a result, a resonance frequency of the third drive arm13increases. On the contrary, when the acceleration α is applied in the −Z axis direction, in other words, when inertial force works in the +Z axis direction, compressive stress works in the third drive arm13that is disposed between the third and fourth concave sections15′c,15′dand the resonance frequency of the third drive arm13decreases.

Since the resonance frequency of the first drive arm10changes in an opposite way to that of the third drive arm13when the acceleration α is applied, the distortion distribution in the base part6awhich is caused by the oscillation of the first drive arm10and the distortion distribution in the base part6awhich is caused by the oscillation of the third drive arm13becomes asymmetrical with respect to the center line extending from the center of the sensing arm12. In other words, the oscillating system is imbalanced, the distortion spreads to the sensing arm12, and the one-end fixed flexural vibration is excited. An amplitude of the one-end fixed flexural vibration monotonically increases depending on the magnitude of the acceleration applied whereas the amount of the electric charge excited in the sensing arm12is proportional to the amplitude of the flexural vibration. By using these two relations, it is possible to calculate the magnitude of the applied acceleration from the amount of the electric charge that is picked up by the electrode. Since the first and third drive arms10,13oscillate in an opposite phase each other according to this embodiment, the oscillating system is more easily imbalanced by application of the acceleration, therefore the sensitivity for detecting the acceleration is improved compared with the acceleration sensing device1of the first embodiment.

The first and second concave sections15′a,15′bare disposed on the front face of the base parts6a,6band in the base edge parts10a,10brespectively. The third and fourth concave sections15′c,15′dare disposed on the back face of the base parts6a,6band in the base edge parts13a,13b. These concave sections are disposed in a point-symmetrical manner each other. Thereby a different stress (the compressive stress or the extensional stress) works in the first and third drive arms depending on the direction in which the acceleration α is applied, and frequency change in the arms also differ each other. The oscillation excited in the sensing arm12depends on a dimensional size of the sensing arm12, and a phase difference between the sensing arm12and the drive arms10,13depends on the direction of the acceleration. Consequently the value of the acceleration can be obtained from the amount of electric charge in the sensing arm12, and the direction of the applied acceleration can be detected from the phase.

The acceleration sensing devices1,2according to the first and second embodiments has the outer frame5thereby the deformation in the X and Y axis directions is very small. In other words, the sensitivity of the acceleration in the X and Y axis directions (sensitivity of the other axis) is very small.

The piezoelectric substrate is used to form the acceleration sensing devices1,2in the above-described embodiments. A piezoelectric material for the piezoelectric substrate includes crystal, lithium tantalate, lithium niobate, langasite and the like.

Sectional shapes of the first, second and third drive arms can be any shape other than the rectangular shape. For example, it can be an H shape which is adopted for a tuning fork oscillator in order to increase oscillation efficiency.

Moreover, the contour vibration bodies1a,2acan be made of metal, glass or the like, and a piezoelectric ceramic material can be jointed so as to form the acceleration sensing device.

According to the embodiments described above, the first and second arms10,11are provided at the base parts6a,6b, and the sensing arm12is disposed therebetween. The first and second drive arms are excited to oscillate at the same frequency but in an opposite phase thereby no vibration is excited in the sensing arm when no acceleration is applied but a flexural vibration is generated in the sensing arm when acceleration is applied. Consequently electric charge is excited and the value of the applied acceleration can be obtained. An advantage of the embodiments is that the sensitivity detecting acceleration is high because the outer frame5is provided and sensitivities in other axes are suppressed.

Moreover, the first and second drive arms10,11and the sensing arm12are disposed at the base parts6a,6bso as to form a single body there, and the concave sections15a,15bare provided at the base edge parts15a,15bof the first drive arm. Thereby a compressive stress or an extensional (tensile) stress is efficiently applied to the first drive arm and the detection sensitivity of the acceleration sensing device can be enhanced.

Furthermore, the first and second drive arms10,11are oscillated in an opposite phase each other so that oscillation is not excited in the sensing arm when no acceleration is applied. When acceleration is applied, the oscillating system is imbalanced and oscillation is excited in the sensing arm. As a result, it is possible to obtain the value of the acceleration from electric charge. In addition, there is an advantageous effect that the detection sensitivity at the time when acceleration is applied can be enhanced.

Moreover, the first and third drive arms10,13are provided in a both-ends supported manner between the base parts6a,6bof the outer frame5, the sensing arm12is held at the base part6ain the cantilever manner, the concave section is provided in the base edge parts and on one face of the first drive arm, the other concave section is provided in the base edge parts and on the other face of the third drive arm, these concave sections are disposed in the point-symmetrical manner, and the first and third drive arms are excited to oscillate in an opposite phase each other. Thereby oscillation is not generated when no acceleration is applied. When acceleration is applied, the resonance frequency of the first drive arm changes in an opposite way to that of the third drive arm therefore the oscillating system can be efficiently imbalanced. Moreover, it is possible to increase the detection sensitivity of acceleration.

Moreover, the both ends of the first and third drive arms10,13are held by the base parts6a,6bin the both-ends supported manner, the sensing arm12is held by the base part6ain the cantilever manner so as to form a single body there, the concave section is provided in the both base edge parts of the first drive arm and on one face, the other concave section is provided in the both base edge parts of the third drive arm and on the other face. Thereby an opposite stress is applied to the first and second drive arms each other and the oscillating system is imbalanced efficiently. Therefore there is an advantageous effect that the detection sensitivity of the acceleration sensing device can be increased.

Furthermore, the first and second drive arms10,11are oscillated in an opposite phase each other so that oscillation is not excited in the sensing arm when no acceleration is applied. When acceleration is applied, the oscillating system is imbalanced and oscillation is excited in the sensing arm. As a result, it is possible to obtain the value of the acceleration from electric charge that is excited by the oscillation of the sensing arm. In addition, frequency of the first drive arm changes in an opposite direction to that of the third drive arm therefore it is possible to determine the direction in which the acceleration is applied.