Acceleration detection device

Abstract: An acceleration detection device (1) is provided which is capable of detecting accelerations in two or more directions. The base end of a beam (4) is fixed to a support section on a board (3). A weight (5) is provided in the front end of the beam (4) which is formed so as to extend horizontally along the board surface with a gap with it and the surface of the board (3). The position of the center of gravity G of the weight (5) is set at a position spaced perpendicularly apart from the center axis of the beam (4) in order that the inertial moment acts when an acceleration along the length (in the X direction) of the beam (4) is applied. The front end surface of the weight (5) is formed into an upright surface of a movable electrode (7), and a fixed electrode (8) is formed at a position facing the movable electrode (7) with a gap therebetween. When an acceleration in the X or Y direction is applied, the beam (4) is flexed upwardly or downwardly in proportion to the magnitude of the acceleration, causing the weight (5) to be inclined upwardly or downwardly while being displaced upwardly or downwardly. Thus, the electrostatic capacity between the movable electrode (7) and the fixed electrode (8) increases or decreases. The magnitude of the acceleration can be detected on the basis of the amount of such change of the electrostatic capacity.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an acceleration detection device for use in an acceleration detection sensor. 2. Description of the Related Art Fig. 9 shows an example of a conventional acceleration detection device 18. The acceleration detection device 18 comprises a board 20, a support section 25, a beam 21 in the shape of a cantilever, a weight 22, movable electrodes 23a and 23b, and fixed electrodes 24a and 24b. Referring to Fig. 9, the support section 25 is securedly formed on the board surface of the board 20, with the base end of the beam 21 being connected to the support section 25. The beam 21 is formed so as to extend horizontally along the board surface with a gap between it and the board surface of the board 20, with the weight 22 being provided at the extension end of the beam 21. The weight 22 is formed symmetrically between the upper and lower halves with respect to the center axis along the length of the beam 21, with the position of the center of gravity G of the weight 22 at the center axis. The movable electrode 23a is formed on the surface of weight 22 facing the board, and the movable electrode 23b is formed on the opposite surface of weight 22. Further, the fixed electrodes 24a and 24b are provided which face the movable electrodes 23a and 23b, respectively, each with a gap therebetween. Connected to the movable electrodes 23a and 23b and the fixed electrodes 24a and 24b are detection means (not shown) for performing detection by applying a voltage between the movable electrodes 23a and 23b and the fixed electrodes 24a and 24b which face each other and by converting the electrostatic capacity between the movable electrodes 23a and 23b and the fixed electrodes 24a and 24b into a voltage. The acceleration detection device 18 constructed as described above is designed to detect an acceleration in a direction (in the Y direction in the example shown in the figure) perpendicular to the board surface of the board 20. When an upward and downward acceleration (an upward and downward acceleration in the Y direction) in the direction shown in Fig. 9, perpendicular to the surface of the board 20, is applied, an upward and downward inertial force in the Y direction corresponding to the direction and the magnitude of the acceleration occurs (to be specific, when an acceleration occurs upwardly, the inertial force occurs downwardly; when, on the contrary, an acceleration occurs downwardly, the inertial force occurs upwardly). This inertial force causes the beam 21 to be flexibly deformed, causing the weight 22 to be displaced. That is, the movable electrodes 23a and 23b are displaced integrally with the weight 22, and the electrode-to- electrode distance between the movable electrodes 23a and 23b and the fixed electrodes 24a and 24b varies. For this reason, the electrostatic capacity between the movable electrodes 23a and 23b and the fixed electrodes 24a and 24b varies. The magnitude of the upward and downward acceleration or the like is detected on the basis of the amount of such change of the electrostatic capacity. However, in the acceleration detection device 18 having the above-described construction, since the position of the gravity G of the weight 22 is on the center axis of the beam 21, when an acceleration along the length (in the X direction) of the beam 21 is applied, an inertial moment does not act on the beam 21, and the weight 22 is not displaced. Therefore, the electrostatic capacity between the movable electrodes 23a and 23b and the fixed electrodes 24a and 24b does not vary, and therefore, an acceleration in the X direction cannot be detected. That is, in the acceleration detection device 18 constructed as described above, only the acceleration in the Y direction perpendicular to the surface of the board 20 can be detected. Of course, it is not possible to detect the magnitude of acceleration in two or more directions. EP 0369352A1 concerns a capacitance type accelerometer which has a first silicon element comprising a silicon beam holding a movable electrode in a cantilever fashion. The silicon beam is not disposed on the axis of gravity of the movable electrode. Further, two silicon plates are provided in between which the silicon beam and the movable electrodes are provided, and the two silicon plates serve as the fixed electrodes of the accelerometer. WO 95/02431 concerns a multi-axis physical activity sensor for use with the rate responsive pacemaker. The sensor includes a cantilever beam having a first transducing layer laminated to a first surface of the cantilever beam and a second transducing layer laminated to a second surface of the cantilever beam. An offset mask is fixed to a free end of the cantilever beam to enhance the magnitude of the deflections, thereby causing the sensor to provide a stronger output signal. It is the object of the present invention to provide an acceleration detection device which is capable of detecting acceleration in two or more directions through the use of a single acceleration detection device. This object is achieved by an acceleration detection device in accordance with claim 1. According to the present invention there is provided an acceleration detection device, wherein a cantilever which extends horizontally along a surface of a board is formed on the board with a gap between it and the surface of the board, with one end of the cantilever being fixed onto the board, and a weight is provided in the front end of the cantilever. The front-end surface of the weight is formed into an upright surface of a movable electrode, a fixed electrode is provided at a facing position with a gap between it and the surface of the movable electrode of the weight, and the position of the gravity of the weight is set at a position spaced perpendicularly apart from the center axis along the length of the cantilever so that the beam can be flexibly deformed when an acceleration along the length of the cantilever is applied. According to a further aspect of the present invention, there is provided an acceleration detection device wherein an upright surface of the movable electrode is formed also on the back-end surface in the weight provided on the front-end side of the cantilever, a fixed electrode is provided each at a position facing, with a gap, the surface of the movable electrode on the front-end surface of the weight and facing, with a gap, the surface of the back-end surface of the weight. According to a further aspect of the present invention, there is provided an acceleration detection device, wherein two or more acceleration detection devices are formed. According to a further aspect of the present invention, there is provided an acceleration detection device, wherein when the number of two or more acceleration detection devices is set at N, the respective acceleration detection devices are arranged mutually at a plane angle of 360°/N. In the present invention constructed as described above, since the position of the gravity of the weight is set at a position spaced perpendicularly apart from the center axis along the length of the cantilever, when an acceleration along the length (e.g., in the X direction) of the cantilever is applied, an inertial moment acts on the beam in proportion to the magnitude of the acceleration, and the beam is flexibly deformed in a direction (e.g., in the Y direction) perpendicular to the board surface, causing the weight to be displaced. That is, the movable electrode is displaced, and the electrode-to- electrode distance between the movable electrode and the fixed electrode varies. In correspondence with the amount of the change of the electrode-to-electrode distance, the electrostatic capacity between the movable electrode and the fixed electrode increases or decreases, and thus the magnitude of the acceleration can be detected on the basis of the amount of such change of the electrostatic capacity. Further, when an acceleration in a direction (in the Y direction) perpendicular to the board surface is applied, an inertial force proportional to the magnitude of the acceleration acts on the cantilever and the weight, causing the beam to be flexibly deformed in a direction (in the Y direction) perpendicular to the board surface and causes the weight to be displaced. Thereupon, in the same way as that stated above, the electrode-to-electrode distance between the movable electrode and the fixed electrode varies, and the electrostatic capacity between the movable electrode and the fixed electrode increases or decreases. Thus, the magnitude of the acceleration can be detected on the basis of the amount of such change of the electrostatic capacity. The above and further objects, aspects and novel features of the invention will become more apparent from the following detailed description when read in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is an illustration of a first embodiment of the present invention; Figs. 2A and 2B are illustrations of a second embodiment of the present invention; Fig. 3 is an illustration of a third embodiment of the present invention; Fig. 4 is an illustration of a fourth embodiment of the present invention; Figs. 5A and 5B are illustrative examples of arrangements using two acceleration detection devices; Figs. 6A and 6B are illustrative examples of arrangements using three acceleration detection devices; Fig. 7 is an illustration of another embodiment of the present invention; Fig. 8 is a circuit diagram illustrating an example of connection between capacitors having electrostatic capacities C 1 and C 2 when a voltage corresponding to the sum of or the difference between the electrostatic capacities C 1 and C 2 is detected; and Fig. 9 shows a conventional acceleration detection device. DESCRIPTION OF THE PREFERRED EMBODIMENTS The preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Fig. 1 shows the main components of an acceleration detection device 1 according to a first embodiment of the present invention. The acceleration detection device 1 comprises a board 3, a support section 6, a beam 4 in the shape of a cantilever, a weight 5, a movable electrode 7, and a fixed electrode 8. Referring to Fig. 1, the support section 6 made of SiO 2 or the like is securedly formed on the board surface of the board 3 made of Si or the like, with the base end of the beam 4 made of Si or the like being fixed to the support section 6. The beam 4 is formed so as to extend horizontally (in the X direction in the example shown in the figure) along the board surface with a gap between it and the board surface, with the weight 5 laminated with a conductor metal of an Fe-Ni alloy or the like being provided in the extension end of the beam 4. The position of the center of gravity G of the weight 5 is set at a position which is spaced perpendicularly (upwardly) apart by Δy shown in Fig. 1 from the center axis along the length of the beam 4 so that when an acceleration in the X direction, which is along the length of the beam 4 is applied, the beam 4 is flexibly deformed in a direction (in the Y direction in the example shown in the figure) perpendicular to the board surface due to the inertial moment. The front-end surface of the weight 5 is formed into an upright surface of the movable electrode 7, with the fixed electrode 8 being provided at a facing position with a gap between it and the movable electrode 7. The fixed electrode 8 is formed by laminating a conductor metallic layer 14 of an Fe- Ni alloy or the like on where an SiO 2 layer 12 and a polycrystalline Si layer 13 are laminated on the board surface of the board 3, with the surface of the conductor metallic layer 14 facing the movable electrode 7 being formed into a surface of the fixed electrode 8. Connected to the acceleration detection device 1 constructed as described above is signal processing means (not shown) for detecting the electrostatic capacity between the movable electrode 7 and the fixed electrode 8, processing the voltage signal, and determining the magnitude of the acceleration or the like. The acceleration detection device 1 detects the acceleration along the length (in the X direction) of the beam 4 and the acceleration in a direction (in the Y direction) perpendicular to the board surface of the board 3 in the following way. When an acceleration along the length (in the side-to-side X direction) of the beam 4 in the side-to-side direction shown in Fig. 1 is applied to the acceleration detection device 1, as described above, the position of the center of gravity G of the weight 5 is at a position which is spaced upwardly apart by Δy from the center axis along the length of the beam 4. Therefore, an inertial moment acts on the beam 4 in proportion to the acceleration in the aforementioned side-to-side X direction. For this reason, the beam 4 is flexibly deformed in an upward and downward direction (in the upward and downward Y direction) perpendicular to the board surface of the board 3 (to be specific, when an acceleration in a leftward X direction occurs, the beam 4 is flexed downward, and when an acceleration in a rightward X direction occurs, the beam 4 is flexed upward), causing the weight 5 to be displaced vertically. When, for example, the beam 4 is flexibly deformed toward the board 3 side and the weight 5 is displaced downward, as indicated by the chain line of Fig. 1, the weight 5 is inclined to the fixed electrode 8 side. When, on the contrary, the weight 5 is displaced upward, the fixed electrode 8 is inclined in a direction opposite to that described above. Because of this, the electrode-to-electrode gap (distance) between the movable electrode 7 and the fixed electrode 8 varies in proportion to the magnitude of the acceleration in the side-to-side X direction, causing the electrostatic capacity between the movable electrode 7 and the fixed electrode 8 to increase or decrease. The magnitude of the acceleration or the like is detected on the basis of the amount of such change of the electrostatic capacity. Further, when an upward and downward (the upward and downward Y direction) acceleration perpendicular to the surface of the board 3 is applied, an upward and downward inertial force in the Y direction occurs in proportion to the magnitude of the acceleration, the beam 4 is flexibly deformed due to this inertial force. In the same way as that described above, the weight 5 is displaced vertically and inclined. The electrode-to-electrode gap between the movable electrode 7 and the fixed electrode 8 varies, causing the electrostatic capacity between the movable electrode 7 and the fixed electrode 8 to increase or decrease. The magnitude of the acceleration or the like is detected on the basis of the amount of such change of the electrostatic capacity. According to this embodiment, since the weight 5 is formed so that the position of the center of gravity G of the weight 5 is at a position spaced vertically (upwardly) apart from the center axis along the length of the beam 4, when an acceleration along the length (in the X direction) of the beam 4 is applied, an inertial moment acts on the beam 4 in proportion to the magnitude of the acceleration, the beam 4 is flexibly deformed, and the weight 5 is displaced vertically and inclined. For this reason, the electrode-to-electrode distance (gap) between the movable electrode 7 and the fixed electrode 8 varies, causing the electrostatic capacity between the movable electrode 7 and the fixed electrode 8 to vary. Thus, the magnitude of the acceleration can also be detected for the acceleration in the X direction on the basis of the amount of such change of the electrostatic capacity. Of course, since the acceleration in the Y direction perpendicular to the board 3 can be detected, it becomes possible to detect the magnitude of the acceleration in both the X and Y directions. A second embodiment of the present invention will now be described with reference to Figs. 2A and 2B. Those components in the second embodiment which are the same or perform the same function as those components in the first embodiment are given the same reference numerals or the same reference numerals and letter suffixes, and a description thereof is omitted. A feature of this embodiment which is different from the first embodiment is that, as shown in Fig. 2A, the back-end surface of the weight 5 is also formed into the upright electrode surface of the movable electrode 7B, and the fixed electrode 8B which faces the movable electrode 7B is provided with a gap therebetween, and the magnitude of the acceleration can be detected with a higher degree of accuracy. The fixed electrode 8B is formed by laminating a conductor metallic layer 11 of an Fe-Ni alloy or the like on the support section 6, and the surface of the conductor metallic layer 11 facing the movable electrode 7B is formed into the surface of the fixed electrode 8B. The construction other than that described above is substantially the same as that of the above- described first embodiment. In this embodiment, an electrostatic capacity C 1 between the movable electrode 7A and the fixed electrode 8A, and an electrostatic capacity C 2 between the movable electrode 7B and the fixed electrode 8B are substantially equal to each other when an acceleration is not applied, and C 1 = C 2 = C 0 is thereby satisfied. Further, when an acceleration in the X or Y direction is applied, in a case in which, for example, C 1 is increased by ΔC with respect to C 0 , C 2 is decreased by ΔC' (ΔC' is almost equal to ΔC). In the above way, the electrostatic capacities C 1 and C 2 are formed so as to vary by nearly the same degree. Further, as shown in Fig. 2B, a tunnel hole 10 for passing the beam 4 therethrough is provided in the central portion of the conductor metallic layer 11. The gap between the ceiling surface of the tunnel hole 10 and the beam 4 is opened to such an extent so as not to obstruct the flexing and deformation of the beam 4 due to the acceleration. A conductor pattern 16 for applying a voltage to the movable electrodes 7A and 7B of the weight 5 is formed on the beam 4, as shown in Figs. 2A and 2B. The thickness of the conductor pattern 16 is extremely thin, and they are sufficiently apart so that the electrostatic capacity between the conductor pattern 16 and the conductor metallic layer 11 becomes small. In the acceleration detection device 1 having the construction shown in Fig. 2A, as a result of the application of an acceleration along the length (in the X direction) of the beam 4 or in a direction (in the Y direction) perpendicular to the board surface of the board 3, in the same way as in the first embodiment, the beam 4 is flexibly deformed vertically in proportion to the magnitude of the acceleration, causing the weight 5 to be inclined while being displaced vertically. When the weight 5 is displaced downward, as described in the first embodiment, the weight 5 is inclined toward the fixed electrode 8A side shown in Fig. 2A, the electrode-to-electrode gap between the movable electrode 7A and the fixed electrode 8A becomes narrower, and on the contrary, the electrode-to-electrode gap between the movable electrode 7B and the fixed electrode 8B is widened. For this reason, the electrostatic capacity C 1 between the movable electrode 7A and the fixed electrode 8A, for example, increases by ΔC with respect to the C 0 . On the contrary, the electrostatic capacity C 2 between the movable electrode 7B and the fixed electrode 8B, for example, decreases by ΔC' (ΔC' is nearly equal to ΔC) with respect to the C 0 . By subtracting C 2 = C 0 - ΔC' from the C 1 = C 0 + ΔC (or by subtracting C 1 from C 2 ), the following is obtained: C 1 - C 2 = (C 0 + ΔC) - (C 0 - ΔC') = ΔC + ΔC'. The amount of change of the electrostatic capacity can be easily obtained. Thus, the magnitude of the acceleration can be detected on the basis of the amount of such change of the electrostatic capacity. Since in the above calculated electrostatic capacity ΔC + ΔC', ΔC' is nearly equal to ΔC, a rearrangement can be made as: ΔC + ΔC' ≈ 2ΔC . That is, an amount of change of the electrostatic capacity substantially twice as large can be obtained than a case in which only a pair of a movable electrode and a fixed electrode are provided. Thus, it is possible to detect the magnitude of the acceleration with a high degree of accuracy. Further, when the weight 5 is displaced upward, the weight 5 is inclined toward the fixed electrode 8B side. In contrast to that described above, the electrode-to-electrode gap between the movable electrode 7A and the fixed electrode 8A is widened, and the electrostatic capacity C 1 between the movable electrode 7A and the fixed electrode 8A decreases with respect to C 0 . On the contrary, the electrode-to-electrode gap between the movable electrode 7B and the fixed electrode 8B becomes narrower, and the electrostatic capacity C 2 between the movable electrode 7B and the fixed electrode 8B decreases with respect to C 0 . Similarly to that described above, the magnitude of the acceleration can be detected with a high degree of accuracy on the basis of the amount of difference between the electrostatic capacities C 1 and C 2 . According to this embodiment, an acceleration in both the X direction and the Y direction can be detected in the same way as in the first embodiment, the back-end surface of the weight 5 provided on the front-end surface side of the beam 4 is formed into the surface of the movable electrode 7B, and the fixed electrode 8B which faces the movable electrode 7B with a gap therebetween is provided. Therefore, both the electrostatic capacity C 1 between the movable electrode 7A and the fixed electrode 8A and the electrostatic capacity C 2 between the movable electrode 7B and the fixed electrode 8B, which electrostatic capacities are increased or decreased due to the acceleration in the X or Y direction, can be obtained, and thus it becomes possible to detect the magnitude of the acceleration in both the X and Y directions with a higher degree of accuracy on the basis of the amount of such change of the electrostatic capacity. In the meantime, the following sometimes occurs. When an acceleration in a direction (in the Z direction) perpendicular to the direction along the length of the beam 4 along the surface of the board 3 is applied, the beam 4 swings slightly in the Z direction due to the inertial force, causing the weight 5 to be displaced in the Z direction. Therefore, the electrode-to-electrode area between the movable electrodes 7A and 7B and the fixed electrodes 8A and 8B decreases, and the electrostatic capacities C 1 and C 2 decrease each by nearly the same degree. However, as in this embodiment, the amount of the change of the electrostatic capacity due to the acceleration in the Z direction is removed by determining the amount of difference between the electrostatic capacities C 1 and C 2 , for example, when C 1 and C 2 each decrease by ΔC due to the acceleration in the Z direction, the following is satisfied: C 1 - C 2 = (C 0 - ΔC) - (C 0 - ΔC) = 0. For this reason, it is possible to detect only the acceleration in the X and Y directions with a high degree of accuracy. Further, since the fixed electrodes 8A and 8B which face each other are provided on the front-end surface and the back-end surface of the weight 5 each with a gap therebetween, even if an extremely large acceleration is applied and the weight 5 is going to be displaced excessively, the fixed electrodes 8A and 8B act as stoppers for the weight 5, thus preventing the weight 5 from being displaced excessively. That is, the beam 4 is prevented from being flexibly deformed excessively, thereby preventing the beam 4 from being damaged. Thus, it is possible to increase the resistance to impact of the acceleration detection device 1. A third embodiment of the present invention will now be described with reference to Fig. 3. Those components in the third embodiment which are the same as or perform the same function as those components in the first embodiment are given the same reference numerals or are given the same reference numerals followed by letter suffixes, and a description thereof is omitted. A feature of this embodiment is that two acceleration detection devices 1A and 1B which are the same as the acceleration detection device 1 described in the first embodiment are provided. As shown in Fig.3, the acceleration detection devices 1A and 1B are provided symmetrically with respect to the symmetry reference plane perpendicular to the board 3 between conductor metallic layers 14A and 14B with the conductor metallic layer 14A and 14B facing each other. The other construction is basically the same as that of the first embodiment. In this embodiment, the electrostatic capacity C 1 between the movable electrode 7A and the fixed electrode 8A of the acceleration detection device 1A and the electrostatic capacity C 2 between the movable electrode 7B and the fixed electrode 8B of the acceleration detection device 1B are almost equal to each other when no acceleration is applied, and C 1 = C 2 = C 0 is thereby satisfied. When an acceleration in the X or Y direction is applied, the electrostatic capacities C 1 and C 2 are formed so as to vary each by nearly the same degree, as described below. When a rightward acceleration shown in Fig. 3 along the length (in the rightward X direction) of the beam 4 is applied, on the acceleration detection device 1A side, the beam 4A is flexibly deformed upwardly in proportion to the magnitude of the acceleration due to the inertial moment, and the weight 5A is inclined toward the left side while being displaced upwardly. On the acceleration detection device 1B side, the beam 4B is flexibly deformed downwardly, and the weight 5B is inclined toward the left side while being displaced downwardly. For this reason, the electrode-to-electrode gap between the movable electrode 7A and the fixed electrode 8A becomes wider, and the electrostatic capacity C 1 between the movable electrode 7A and the fixed electrode 8A, for example, decreases by ΔC with respect to C 0 , as shown in Table 1. In contrast, the electrode-to-electrode gap between the movable electrode 7B and the fixed electrode 8B becomes narrower, and the electrostatic capacity C 2 between the movable electrode 7B and the fixed electrode 8B, for example, increases by ΔC' with respect to C 0 (ΔC' is almost equal to ΔC). Direction of Acceleration X Direction Y Direction Rightward Leftward Upward Downward Electrostatic Capacity C 1 C 0 -ΔC C 0 +ΔC C 0 +ΔC C 0 -ΔC C 2 C 0 +ΔC' C 0 -ΔC' C 0 +ΔC' C 0 -ΔC' C 1 -C 2 -2ΔC (ΔC≈ΔC') 2ΔC (ΔC≈ΔC') 0 (ΔC≈ΔC') 0 (ΔC≈ΔC') Further, when a leftward acceleration in the X direction is applied, the beam 4A of the acceleration detection device 1A is flexibly deformed downwardly in proportion to the magnitude of the acceleration due to the inertial moment, causing the weight 5A to be inclined toward the right side while being displaced downwardly. In contrast, the beam 4B of the acceleration detection device 1B is flexibly deformed upwardly, causing the weight 5B to be inclined toward the right side while being displaced upwardly. Therefore, the electrostatic capacity C 1 , for example, increases by ΔC with respect to C 0 , as shown in Table 1. In contrast, the electrostatic capacity C 2 decreases by ΔC', which is almost equal to ΔC, with respect to C 0 . Further, when an upward and downward acceleration in a direction (in the upward and downward Y direction) perpendicular to the board 3 is applied, an upward and downward inertial force corresponding to the magnitude of the acceleration occurs. In the same way as that described above, the beams 4A and 4B are flexibly deformed, causing the weights 5A and 5B to be inclined while being displaced. As a result, the electrostatic capacities C 1 and C 2 increase or decrease by the same degree with respect to C 0 , as shown in Table 1. When the electrostatic capacities C 1 and C 2 are detected, the magnitude of the acceleration in both the X and Y directions can be detected on the basis of the amount of the change of C 1 and C 2 . Further, when the amount of difference such that C 2 is subtracted from C 1 (or the amount of difference such that C 1 is subtracted from C 2 ) is determined, as shown in Table 1, only the amount of change of C 1 and C 2 due to the acceleration in the X direction can be obtained. Thus, it is possible to detect only the magnitude of the acceleration in the X direction. According to this embodiment, it is possible to detect the acceleration in both the X and Y directions in the same way as in the first embodiment. Further, since both the acceleration detection devices 1A and 1B are provided, and the devices are provided symmetrically with respect to the symmetry reference plane, when an acceleration in the X direction is applied, the flexing direction of the beam 4A of the acceleration detection device 1A becomes opposite to that of the beam 4B of the acceleration detection device 1B. As a result, the capacities C 1 and C 2 increase or decrease in such a way that when the electrostatic capacity C 1 increases, the capacity C 2 decreases. For this reason, the determination of the amount of difference between the electrostatic capacities C 1 and C 2 makes it possible to easily detect only the magnitude of the acceleration in the X direction. That is, not only the magnitude of the acceleration, but also the direction of the acceleration can be easily detected. Of course, since an amount of change of the electrostatic capacity larger than in a case in which a pair of the movable electrode 7 and the fixed electrode 8 which face each other are provided can be obtained, it is possible to detect the magnitude of the acceleration with a higher degree of accuracy. A fourth embodiment of the present invention will now be described with reference to Fig. 4. Those components in the fourth embodiment which are the same as or perform the same function as those components in the third embodiment are given the same reference numerals or the same reference numerals with letter suffixes, and a description thereof is omitted. A feature of this embodiment is that two acceleration detection device 1 shown in the second embodiment are provided, and as shown in Fig. 4, the acceleration detection devices 1A and 1B are formed symmetrically with respect to the symmetry reference plane perpendicular to the board 3 between the conductor metallic layers 14 with the conductor metallic layers 14 facing each other. The other construction is basically the same as that of the third embodiment. In this embodiment, as shown in Fig. 4, four pairs of the movable electrodes 7A and 7B and the fixed electrodes 8A and 8B which face each other are formed. The electrostatic capacities C 11 , C 12 , C 21 and C 22 which are generated between those movable electrodes 7A and 7B and fixed electrodes 8A and 8B are almost equal to each other when an acceleration is not applied, and C 11 = C 12 = C 21 = C 22 = C 0 is thereby satisfied. When an acceleration in the X or Y direction is applied, as shown in Table 2, electrostatic capacities C 11 , C 12 , C 21 and C 22 are formed in such a way that the electrostatic capacities vary by nearly the same amount. Direction of Acceleration X Direction Y Direction Rightward Leftward Upward Downward Elektrostatic Capacity C 11 C 0 -ΔC C 0 +ΔC C 0 +ΔC C 0 -ΔC C 12 C 0 +ΔC' C 0 -ΔC' C 0 -ΔC' C 0 +ΔC' C 21 C 0 +ΔC' C 0 -ΔC' C 0 +ΔC C 0 -ΔC C 22 C 0 -ΔC C 0 +ΔC C 0 -ΔC' C 0 +ΔC' C 11 -C 12 -ΔC-ΔC' ΔC+ΔC' ΔC+ΔC' -ΔC-ΔC' C 21 -C 22 ΔC+ΔC' -ΔC-ΔC' ΔC+ΔC' -ΔC-ΔC' (C 11 -C 12 ) +(C 21 -C 22 ) 0 0 2(ΔC + ΔC') -2(ΔC + ΔC') (C 11 -C 12 ) -(C 21 -C 22 ) -2(ΔC + ΔC') 2(ΔC + ΔC') 0 0 If, for example, a rightward acceleration along the length (in the rightward X direction) of the beam 4 is applied, as described in the third embodiment, the weight 5A of the acceleration detection device 1A is inclined to the left while being displaced upwardly in proportion to the magnitude of the acceleration due to the inertial moment. In contrast, the weight 5B of the acceleration detection device 1B is inclined to the left while being displaced downwardly. As shown in Table 2, the electrostatic capacity C 11 , for example, decreases by ΔC with respect to C 0 , the C 12 increases by ΔC', which is almost the same as ΔC, with respect to C 0 , the C 21 increases by ΔC' with respect to C 0 , and the C 22 decreases by ΔC with respect to C 0 . Further, when a downward acceleration in a direction (in the downward Y direction) perpendicular to the board 3 is applied, the weight 5A is inclined to the left while being displaced upwardly in proportion to the magnitude of the acceleration due to the inertial force, and the weight 5B is inclined to the right while being displaced upwardly. As shown in Table 2, the electrostatic capacity C 11 , for example, decreases by ΔC with respect to C 0 , the C 12 increases by ΔC' with respect to C 0 , the C 21 decreases by ΔC with respect to C 0 , and the C 22 increases by ΔC' with respect to C 0 . In the same way as in the second embodiment, when the electrostatic capacities C 11 , C 12 , C 21 and C 22 are detected, and the C 12 is subtracted from the C 11 (or the C 11 is subtracted from the C 12 ), or the C 22 is subtracted from the C 21 (or the C 21 is subtracted from the C 22 ), as shown in Table 2, the amount of change of the electrostatic capacity can be easily determined. Thus, the magnitude of the acceleration or the like can be detected on the basis of the amount of such changes. When the sum of the amount of difference between the C 11 and C 12 , and the amount of difference between the C 21 and C 22 is determined, as shown in Table 2, only the amount of change of the electrostatic capacity due to the acceleration in the Y direction can be detected. Therefore, only the magnitude of the acceleration in the Y direction can be detected. Further, when the difference obtained by subtracting the amount of difference between C 21 and C 22 from the amount of difference between C 11 and C 12 is determined, only the magnitude of the acceleration in the X direction can be detected. According to this embodiment, the same excellent advantages as those of the second embodiment can be obtained. Further, since two acceleration detection devices 1 shown in the second embodiment are provided, and those acceleration detection devices 1A and 1B are provided symmetrically with respect to the symmetry reference plane, the flexing directions of the beams 4A and 4B are opposite to each other. As a result, when the difference between the amount of difference between the electrostatic capacities C 11 and C 12 and the amount of difference between the electrostatic capacities C 21 and C 22 is determined, only the magnitude of the acceleration along the length (in the X direction) of the beam 4 can be detected. Further, by determining the sum of the amount of difference between the electrostatic capacities C 11 and C 12 and the amount of difference between the electrostatic capacities C 21 and C 22 is determined, only the magnitude of the acceleration in a direction (in the Y direction) perpendicular to the board 3 can be detected. That is, it is possible to detect the magnitude of the acceleration and the direction of the acceleration with a higher degree of accuracy. Of course, since four pairs of the movable electrodes 7a and 7b and the fixed electrodes 8a and 8b are provided, an amount of an even larger change of the electrostatic capacity can be obtained, and thus the magnitude of the acceleration or the like can be detected with high accuracy. The present invention is not limited to each of the above-described embodiments, and various modifications can be made. For example, in the first, second and third embodiments, a pair of the movable electrodes 7 and the fixed electrodes 8 which are upright with respect to the board 3 are provided. However, as shown in Fig. 7, a pair of a movable electrode 7α and a fixed electrode 8α which are upright with respect to the board 3 may be provided, and a pair of a movable electrode 7β and a fixed electrode 8β which are formed parallel to the surface of the board 3 may be provided. Although in the above-described third and fourth embodiments the acceleration detection devices 1A and 1B are provided so as to be symmetrical with respect to the symmetry reference plane, namely, the center axes thereof along the length of the beam 4 are coaxial, as shown in Figs. 5A and 5B, it may be possible that the acceleration detection devices 1A and 1B are arranged such that the center axes of the beams 4 are parallel to each other, or so that the center axes of the beams 4 are crosswise to each other, as shown in Figs. 6A and 6B. Although the above-described third and fourth embodiments describe an example in which two acceleration detection devices 1 described in the first or second embodiment are provided, it may be possible to provide three or more acceleration detection devices. However, in such a case, i.e., when the number of acceleration detection devices formed is three or more, the respective acceleration detection devices should preferably be arranged mutually at a plane angle of 360°/N, where N>3, because such orientation facilitates the analysis of the electrostatic capacity or the like. Thus, as shown in Figs. 6A and 6B, when three of the acceleration detection devices 1A, 1B and 1C are to be provided, these are preferably provided with the center axes along the length of the beams 4 of the acceleration detection devices 1A to 1C mutually making a plane angle of (360°/3) = 120°. Of course, a plurality of acceleration detection devices may be arranged without being limited to the plane angle of 360°/N. The arrangement of a plurality of acceleration detection devices at a plane angle other than 180° makes it possible to detect acceleration not only in both the X and Y directions, but also in directions other than those directions. In addition, in the second embodiment, when the difference between the electrostatic capacities C 1 and C 2 between the movable electrodes 7A and 7B and the fixed electrodes 8A and 8B is to be determined, C 1 and C 2 are detected, and then the difference between the electrostatic capacities C 1 and C 2 is determined by signal processing. However, it is possible to determine the difference between C 1 and C 2 by incorporating the acceleration detection device into the following detection circuit. For example, when the difference between C 1 and C 2 is to be obtained, two pairs of movable electrodes 7A and 7B and fixed electrodes 8A and 8B shown in Fig. 2A are connected in series as shown in Fig. 8A. By detecting the voltage at the series connection point X, the voltage corresponding to the difference between C 1 and C 2 can be detected. When the sum of C 1 and C 2 is to be obtained, two pairs of movable electrodes 7A and 7B and fixed electrodes 8A and 8B are connected in parallel as shown in Fig. 8B, and thus the voltage corresponding to the sum of C 1 and C 2 can be detected. The sum of and difference between the electrostatic capacities can be determined for the third to fourth embodiments in the same way as that described above. Further, in the second to fourth embodiments, a plurality of pairs of the movable electrodes 7A and 7B and the fixed electrodes 8A and 8B are provided. In the pairs of those movable electrodes 7A and 7B and the fixed electrodes 8A and 8B, all of the respective electrostatic capacities are equal to each other when no acceleration is applied, and when an acceleration is applied in the X or Y direction, the movable electrodes 7A and 7B and the fixed electrodes 8A and 8B are provided in order that the electrostatic capacity varies by nearly the same degree in proportion to the magnitude of the acceleration. However, when no acceleration is applied, the electrostatic capacities of the pair of movable electrodes 7A and 7B and the fixed electrodes 8A and 8B need not be equal to each other, and the electrostatic capacities need not to vary by nearly the same degree. According to the present invention, a weight is provided at the front end of a cantilever and the position of the gravity of this weight is set at a position spaced perpendicularly apart from the center axis along the length of the cantilever so that it can be flexibly deformed when an acceleration is applied along the length of the cantilever. Therefore, not only when an acceleration in a direction perpendicular to the board surface is applied, but also when an acceleration is applied along the length of the cantilever, an inertial moment proportional to the magnitude of the acceleration acts, and the weight is displaced in linkage with the flexing and deformation of the beam. The movable electrode is displaced due to the displacement of the weight, causing the electrode-to-electrode distance between the movable electrode and the fixed electrode to vary and causes the electrostatic capacity between the movable electrode and the fixed electrode to vary. Thus, the magnitude of the acceleration or the like can be detected on the basis of the amount of such change of the electrostatic capacity. That is, the acceleration detection device of the present invention is capable of detecting acceleration in a direction perpendicular to the board surface and in a direction along the length of the beam. In addition, revolutionary advantages can be shown, for example, in a case in which two acceleration detection devices in accordance with the first, second and third aspects of the present invention are formed and the acceleration detection devices are mutually arranged at a plane angle of 180°, it is possible to detect accelerations in the above-described two directions with a higher degree of accuracy, and only the magnitude of the acceleration in one direction perpendicular to the board surface or along the length of the beam can be detected. Further, in a case in which N acceleration detection devices in accordance with the first, second and third aspects of the present invention are formed and mutually arranged at a plane angle of 360°/N, it becomes possible to detect accelerations in two or more directions.

Claims: An acceleration detection device (1), comprising a beam (4) in shape of a cantilever which extends horizontally along a surface of a board (3) and is formed on the board (3) with a gap between it and the surface of the board (3), with one end of the beam (4) being fixed onto the board (3), and a weight (5) provided in the front end of the beam (4), the position of the center of gravity of said weight (5) is set at a position spaced perpendicularly apart from the center axis along the length of the beam (4) so that the beam (4) can be flexibly deformed when an acceleration along the length of the beam (4) is applied, characterized in that a front end surface of the weight is formed into an upright surface of a movable electrode (7; 7α), and a fixed electrode (8; 8α) is provided at a position with a gap between it and the surface of the movable electrode (7; 7α) of the weight (5). An acceleration detection device (1) according to claim 1, wherein an upright surface of the movable electrode (7A, 7B) is formed also on the back end surface of the weight (5) provided on the front end side of the beam (4), a fixed electrode (8A, 8B) is provided each at a position facing, with a gap, the surface of the movable electrode (7A) on the front end surface of the weight (5), and the surface of the movable electrode (7B) on the back end surface of the weight (5). An acceleration detection device (1) according to claim 1 or 2, wherein: a further movable electrode (7β) is mounted on a horizontal surface of the beam (4) and a further fixed electrode (8β) is horizontally mounted on the board (3) opposite to the further movable electrode (7β). Apparatus for detecting acceleration, comprising: a plurality of acceleration detection devices (1) according to any of claims 1 to 3. Apparatus for detecting acceleration according to claim 4, including N acceleration detection devices (1) (N>3), the acceleration detection devices (1) being mutually arranged such that the horizontal axes of their respective beams (4) intersect at plane angles of 360°/N.

Inventors: NEGORO YASUHIRO, NEGORO, YASUHIRO, Negoro, Yasuhiro, Murata Manufact. Co., Ltd.
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1996
Main classification: G01P15
IPC codes: G01P15