Acceleration sensor

Abstract: An acceleration sensor comprising: an acceleration detecting portion (32) comprising a piezoelectric element (31) having electrodes (31a, 31b) on both its surfaces and outputting a signal corresponding to acceleration applied and a supporting base for supporting said piezoelectric element (31), said acceleration detecting portion (32) being provided with a pair of detecting portions (34a, 34b) arranged so as to be brought into contact with said piezoelectric element (31) and spaced apart from each other by a predetermined distance; signal processing means (36) electrically connected to one of the electrodes (31b) of said piezoelectric element (31) and for processing the electric signal outputted from the piezoelectric element (31); and signal outputting means (35) electrically connected to one (34a) of said pair of detecting portions (34a, 34b) so as to cause a predetermined current to flow between said detecting portions and connected to said signal processing means (36) so as to output to the signal processing means (36) a state signal representing the state of the acceleration detecting portion (32); said signal outputting means (35) outputting said state signal in conformity with the variation of the current flowing between the detecting portions (34a, 34b).

Description: The present invention relates generally to an acceleration sensor used with it being incorporated in an air bag device carried by an automobile, and more particularly, to an acceleration sensor comprising a fault diagnostic function. In an air bag device carried by an automobile, the air bag device is operated according to acceleration applied at the time of, for example, the collision. In order to ensure the operation of the air bag device, an acceleration sensor has been conventionally incorporated in the above described air bag device. As this type of acceleration sensor, an acceleration sensor using a piezoelectric element which is deformed in conformity with acceleration applied to output an electric signal has been proposed, as disclosed in, for example, U. S. Patent No. 4, 700, 973. One example of a conventionally known acceleration sensor utilizing a piezoelectric element will be described with reference to Figs. 1 to 3. Fig. 1 is a schematic block diagram for explaining the construction of a conventional acceleration sensor. There is provided a piezoelectric element 1 outputting, when acceleration G is applied, an electric signal corresponding to the acceleration G. Impedance converting means 2 is electrically connected to the piezoelectric element 1. The impedance converting means 2 converts the impedance of the electric signal applied from the piezoelectric element 1. Filtering means 3 comprising a band-pass filter is electrically connected to the impedance converting means 2. In the filtering means 3, an unnecessary signal, that is, an out-of-band signal component is attenuated. Amplifying means 4 is electrically connected to the filtering means 3. In the amplifying means 4, an output signal applied from the filtering means 3 is amplified. This acceleration sensor has the piezoelectric element 1, the impedance converting means 2, the filtering means 3 and the amplifying means 4, and outputs a voltage signal corresponding to the acceleration G from an output terminal B. The voltage signal outputted from the output terminal B of the acceleration sensor is applied to control means 5 comprising a microcomputer arranged outside the acceleration sensor. The control means 5 causes an air bag device for an automobile (not shown) to perform a necessary operation on the basis of the voltage signal applied. Figs. 2 and 3 are respectively a plan sectional view showing the specific construction of the above described acceleration sensor and a cross sectional view taken along a line B - B shown in Fig. 2. In an acceleration sensor 6, a base plate 7 and a cap 8 secured to the upper surface of the base plate 7 constitute a package structure containing a housing space. Within the package structure, a hybrid IC 9 is secured on the base plate 7 using adhesives (not shown). The hybrid IC 9 is used for constructing the impedance converting means 2, the filtering means 3 and the amplifying means 4 described above. A plurality of electrodes 9a to 9f for connection to outer portions are formed on the upper surface of the hybrid IC 9. Each of the electrodes 9a to 9d is electrically connected to a lead terminal 11 by a lead wire 10. A plurality of lead terminals 11 are passed through the base plate 7 and are extended downward. On the other hand, a supporting base 12 is secured on the base plate 7 using adhesives (not shown) beside the hybrid IC 9. A piezoelectric element 1 having electrodes (not shown) on both its major surfaces is secured on the supporting base 12 in a cantilevered shape. The electrode on the upper surface of the piezoelectric element 1 is electrically connected to the electrode 9f by a lead wire 10f, and the electrode on the lower surface of the piezoelectric element 1 is electrically connected to the supporting base 12. In addition, the supporting base 12 is electrically connected to the electrode 9e on the hybrid IC 9 by a lead wire 10e. The above described piezoelectric element 1 comprises one having a series type bimorph structure which is low in pyroelectric voltage. Meanwhile, the above described package structure constituted by the base plate 7 and the cap 8 is hermetically sealed, and an inert gas, for example, nitrogen gas is sealed into the package structure so as to prevent oxidation. When the above described acceleration sensor is incorporated in the air bag device for an automobile, the air bag device is operated in response to an output signal of the acceleration sensor. Accordingly, the acceleration sensor requires very high reliability, and a fault in the acceleration sensor must be quickly found out. Since a fault diagnostic function has not been conventionally added to the acceleration sensor itself, however, the fault diagnosis of the acceleration sensor is generally made by an outer fault diagnostic device which is provided separately from the acceleration sensor. When there occurs a fault such as the cracking of the piezoelectric element 1 in the acceleration sensor or the stripping of the piezoelectric element 1 from the supporting base 12, the fault may not, in some cases, be quickly found out because the acceleration sensor does not have a fault self-diagnostic function. Furthermore, the conventional acceleration sensor 6 shown in Figs. 2 and 3 also have the following disadvantages. That is, when an excessive shock is externally given, there occurs a state where the supporting base 12 fixed to the base plate 7 using adhesives is stripped from the base plate 7 and the piezoelectric element 1, along with the supporting base 12, is not fixed to the other members, as represented by an imaginary line in Fig. 3. Accordingly, acceleration cannot, in some cases, be accurately detected by the piezoelectric element 1. Even if such a fault occurs, the electrical connection between the piezoelectric element 1 and the hybrid IC 9 is ensured through the lead wires 10e and 10f so long as the piezoelectric element 1 is fixed to the supporting base 12, so that the occurrence of the fault may not, in some cases, be detected. Therefore, the present invention has been made so as to overcome the above described disadvantages of the conventional acceleration sensor and has for its object to provide an acceleration sensor capable of reliably detecting the occurrence of a fault. In accordance with an aspect of the present invention, there is provided an acceleration sensor comprising an acceleration detecting portion comprising a piezoelectric element having electrodes on both its major surfaces and outputting an electric signal corresponding to acceleration applied and a supporting base for supporting the above described piezoelectric element, the acceleration detecting portion being further provided with a pair of detecting portions arranged so as to be brought into contact with the piezoelectric element and spaced apart from each other by a predetermined distance, signal processing means electrically connected to one of the electrodes of the piezoelectric element and for processing the signal outputted from the piezoelectric element, and signal outputting means electrically connected to one of the above described pair of detecting portions so as to cause a predetermined current to flow between the above described detecting portions and electrically connected to the above described signal processing means so as to output to the above described signal processing means a state signal representing the state of the acceleration detecting portion, the signal outputting means outputting the above described state signal in conformity with the variation of the current flowing between the detecting portions. In the acceleration sensor provided in accordance with the above described aspect of the present invention, a fault self-diagnostic function is provided, as apparent from the embodiments described later. Even when there occurs a fault such as the cracking of the piezoelectric element or the stripping of the piezoelectric element from a portion to which the piezoelectric element is fixed, therefore, the fault which occurred can be quickly found out. Embodiments of the present invention will become more apparent from the following detailed description. Fig. 1 is a schematic block diagram for explaining the construction of a conventional acceleration sensor; Fig. 2 is a plan sectional view for explaining the construction of the conventional acceleration sensor; Fig. 3 is a cross sectional view taken along a line B - B shown in Fig. 2; Fig. 4 is a schematic block diagram for explaining the construction of an acceleration sensor according to an embodiment of the present invention; Fig. 5 is a circuit diagram for explaining the acceleration sensor according to the embodiment; Fig. 6 is a perspective view for explaining the relationship between a supporting base and a piezoelectric element in the acceleration sensor according to the embodiment; Fig. 7 is a perspective view showing the relationship between a piezoelectric element and a supporting base in a modified embodiment; Fig. 8 is a perspective view showing the relationship between a piezoelectric element and a supporting base in another modified embodiment; Fig. 9A is an electric circuit diagram showing a modified example of signal outputting means; and Fig. 9B is a schematic block diagram for explaining a still other modified example of the embodiment; Fig. 10 is a plan sectional view showing an inventive acceleration sensor; Fig. 11 is a cross sectional view taken along a line A - A shown in Fig. 10; Fig. 12 is a schematic block diagram showing main portions of the conventional acceleration sensor; Fig. 13 is a schematic block diagram showing main portions of the acceleration sensor according to the present invention; Fig. 14 is a diagram showing the temperature dependence of the rate of variation of piezoelectric stress constant of a piezoelectric element and the rate of variation of capacitance thereof; Fig. 15 is an exploded perspective view for explaining the construction of the acceleration sensor according to the present invention; Fig. 16 is a cross sectional view for explaining the construction of the acceleration sensor according to Fig. 15; and Fig. 17 is an exploded perspective view showing the schematic construction of a modified example of the acceleration sensor according to the present invention. Figs. 4 and 5 are respectively a block diagram for explaining the schematic construction of an acceleration sensor according to an embodiment of the present invention and an electric circuit diagram showing one example of the specific circuit arrangement of the acceleration sensor. Referring to Fig. 4, there is provided an acceleration detecting portion 32.in the present embodiment. The acceleration detecting portion 32 comprises a piezoelectric element 31 having electrodes 31a and 31b on both its major surfaces, as shown in Fig. 5. As the piezoelectric element 31, a piezoelectric element having a series type bimorph structure which is low in pyroelectric voltage is preferably used. The piezoelectric element 31 is fixed on a supporting base 33, as schematically shown in Fig. 5. A pair of detecting portions 34a and 34b is formed spaced apart from each other by a predetermined distance on the upper surface of the supporting base 33, and the pair of detecting portions 34a and 34b is electrically connected to the electrode 31a of the piezoelectric element 31. One of the detecting portions 34a is electrically connected to signal outputting means 35 as described hereafter. The other detecting portion 34b is electrically connected to a reference potential. The specific construction of the acceleration detecting portion 32 will be described hereafter with reference to, for example, Fig. 6. As can be seen from Fig. 5, the other electrode 31b of the piezoelectric element 31 is electrically connected to signal processing means 36. That is, as shown in Fig. 4, the acceleration detecting portion 32 is electrically connected to the signal processing means 36 so that an output of the acceleration detecting portion 32 is applied to the signal processing means 36. The signal processing means 36 comprises impedance converting means 37, filtering means 38, and amplifying means 39. The impedance converting means 37 is provided so as to convert the impedance of a voltage signal applied from the acceleration detecting portion 32, the filtering means 38 comprises a band-pass filter and is provided so as to attenuate or remove an out-of-band signal component, and the amplifying means 39 is provided so as to amplify a voltage signal applied from the filtering means 38. The signal outputting means 35 is connected to a power supply voltage Vcc, and is constituted by resistors R 11 and R 12 connected in series to each other. A node 40 between the resistors R 11 and R 12 is electrically connected to the signal processing means 36. The specific construction of the above described acceleration detecting portion 32 will be described with reference to Fig. 6. A supporting base 33 is constituted by a pair of supporting members 33a and 33b arranged spaced apart from each other by a predetermined distance. Detecting portions 34a and 34b are formed on the respective supporting members 33a and 33b by applying electrode films. That is, the pair of detecting portions 34a and 34b is arranged spaced apart from each other by a predetermined distance. A piezoelectric element 31 is fixed on the detecting portions 34a and 34b with one of electrodes 31a on the bottom, and the electrode 31a is electrically connected to the detecting portions 34a and 34b. The signal outputting means 35 is electrically connected to the detecting portion 34a, as shown in Fig. 6, and the detecting portion 34b is connected to a reference potential. A fault diagnostic operation in the acceleration sensor according to the second embodiment will be described with reference to Figs. 5 and 6. In the acceleration sensor according to the embodiment as described above, the signal outputting means 35 is connected in series to a predetermined potential source, and comprises a pair of resistors R 11 and R 12 for dividing the power supply voltage Vcc. In addition, respective one input terminals of the filtering means 38 and the amplifying means 39 are electrically connected to the node 40 between the resistors R 11 and R 12 . On the other hand, one of the above described detecting portions 34a is electrically connected to the downstream side of the resistors R 11 and R 12 . Consequently, a predetermined current I 1 flows from the node 40 to the filtering means 38 and the amplifying means 39, and a predetermined current I 2 flows from the downstream side of the resistors R 11 and R 12 to the detecting portion 34a. The above described signal outputting means 35 may be constructed, as in a modified example shown in Fig. 9A. That is, the signal outputting means 35 may be so constructed that a circuit comprising resistors R 13 to R 15 and a transistor TR 3 is provided in the succeeding stage of the node 40 between the resistors R 11 and R 12 , and a current I 3 corresponding to the predetermined current I 1 from the node 40 is applied to respective one input terminals of the filtering means 38 and the amplifying means 39. Meanwhile, in the above described acceleration detecting portion 32, the detecting portions 34a and 34b are electrically connected to each other by the electrode 31a on the lower surface of the piezoelectric element 31, as apparent from Fig. 6. Consequently, a predetermined current I 2 applied from the above described signal outputting means 35 flows between the detecting portions 34a and 34b. That is, the current I 2 flowing between the detecting portions 34a and 34b is always monitored by the signal processing means 36. If the current I 2 varies, the current I 1 flowing from the node 40 between the resistors R 11 and R 12 varies in conformity with the variation of the current I 2 . Accordingly, in, for example, a case where the piezoelectric element 31 is cracked or the piezoelectric element 31 is stripped from the supporting base 33, the current I 2 flowing between- the detecting portions 34a and 34b varies, so that the current I 2 is reduced or does not flow. As a result, the current I 1 flowing from the node 40 between the resistors R 11 and R 12 varies. Accordingly, a state signal representing the state of the acceleration detecting portion 32, that is, the cracking or the stripping of the piezoelectric element 31 constituting the acceleration detecting portion 32 is outputted from the signal outputting means 35 to the filtering means 38 and the amplifying means 39 constituting the signal processing means 36 in conformity with the variation in value of the above described current I 2 . Consequently, a signal outputted outward from the output terminal B of the acceleration sensor varies, thereby to make it possible to make the diagnosis of a fault in the acceleration sensor by the variation of the output signal in control means 41 connected to the output terminal B. Although in the construction shown in Fig. 6, the supporting base 33 constituting the acceleration detecting portion 32 comprises a pair of supporting members 33a and 33b, and the electrode films constituting the detecting portions 34a and 34b are applied to the upper surfaces of the supporting members 33a and 33b, the detecting portions 34a and 34b may be provided separately from the electrode films formed on the supporting members 33a and 33b. In addition, the detecting -portions 34a and 34b need not be respectively provided between the piezoelectric element 31 and the supporting members 33a and 33b. For example, the detecting portions 34a and 34b may be arranged spaced apart from each other by a predetermined distance in another portion of the piezoelectric element 31. Furthermore, the construction of the acceleration detecting portion 32 is not limited to the construction shown in Fig. 6. For example, the signal processing means 36 and the like in the acceleration sensor according to the present embodiment can be usually incorporated in a single electronic component, for example, a hybrid IC. Consequently, as shown in Fig. 7, the supporting base 33 may be replaced with a hybrid IC 42. In the construction shown in Fig. 7, a notch 42a is formed in the center of the hybrid IC 42, a pair of supporting portions 42b and 42c is formed on both sides thereof, and detecting portions 34a and 34b are formed on the upper surface of the pair of the supporting portions 42b and 42c by applying electrode films. A piezoelectric element 31 is bonded and fixed on the detecting portions 34a and 34b with conductive adhesives. Consequently, an electrode 31a formed on one major surface of the piezoelectric element 31 is electrically connected to the electrode films constituting the detecting portions 34a and 34b. Furthermore, as shown in Fig. 8, a piezoelectric element 31 may be fixed on a supporting base 33 in a cantilevered shape using conductive adhesives. In the construction shown in Fig. 8, a pair of electrode films is formed spaced apart from each other by a predetermined distance on the upper surface of the supporting base 33, thereby to constitute detecting portions 34a and 34b. On the other hand, an electrode 31a on the lower surface of the piezoelectric element 31 is formed in a substantially U shape by cutting away its central part as represented by a broken line in Fig. 8, and both ends of the electrode 31a in a substantially U shape are electrically connected to the detecting portions 34a and 34b, respectively. Although in the foregoing description of the embodiment, the filtering means 38 and the amplifying means 39 constituting the signal processing means 36 are electrically connected to the node 40 between the resistors R 11 and R 12 constituting the signal outputting means 35, the present invention is not limited to the same. That is, as shown in Fig. 9B, a fault in the acceleration detecting portion 32 may be diagnosed by directly connecting the signal outputting means 35 and control means X to each other, applying a current from the above described node 40 to the control means X, and comparing the voltage signal applied from the output terminal B of the acceleration sensor and the state signal applied from the signal outputting means 35 with each other in the control means X. As described in the foregoing, the acceleration sensor according to the embodiment comprises the signal outputting means for outputting to the signal processing means the state signal representing the state of the acceleration detecting portion 32 comprising the piezoelectric element 31 and the supporting base 33, and the pair of detecting portions arranged spaced apart from each other by a predetermined distance in the acceleration detecting portion 32. The signal outputting means is so constructed as to output the above described state signal according to the variation of the current flowing between the detecting portions 34a and 34b. Accordingly, when there occurs a fault such as the cracking of the piezoelectric element constituting the acceleration detecting portion or the stripping of the piezoelectric element from the supporting base, the current flowing between the above described detecting portions 34a and 34b varies, so that the state signal corresponding to the above described variation of the current is outputted from the signal outputting means, thereby to make it possible to quickly find out the fault which occurred. Consequently, it is possible to reliably find out a fault, which has been conventionally difficult to find out, such as the cracking of the piezoelectric element or the stripping of the piezoelectric element from the supporting base. Figs. 10 and 11 are respectively a plan sectional view for explaining the acceleration sensor of the present invention and a schematic cross sectional view taken along a line A - A shown in Fig. 10. It is characterized in combination with claim 1 by a supporting structure of a piezoelectric element. Referring to Figs. 10 and 11, the acceleration sensor 51 in combination with the subject matter of claim 1 comprises a base plate 52 and a cap 53 secured to the upper surface of the base plate 52 at the peripheral edge, and the base plate 52 and the cap 53 constitute a package structure. In a housing space constituted by the base plate 52 and the cap 53, a hybrid IC 54 is fixed on the base plate 52 using adhesives (not shown). The hybrid IC 54 has a structure containing the signal processing means 36. The upper surface of the hybrid IC 54 is made flat, and electrodes 54a to 54f for connection to outer portions are formed on the upper surface. Each of the electrodes 54a to 54d is electrically connected to a lead terminal 56 by a lead wire 55. The lead terminal 56 is passed through the base plate 52 and is extended downward from the base plate 52 in a state where it is electrically insulated from the base plate 52. On the other hand, the electrode 54e is electrically connected to an electrode 31b on the upper surface of a piezoelectric element 31 by a lead wire 57. In addition, the piezoelectric element 31 is directly secured to the upper surface of the hybrid IC 54 with conductive adhesives, and an electrode 31a on the lower surface of the piezoelectric element 31 is electrically connected to the electrode 54f with the above described conductive adhesives. The area of the bottom surface of the hybrid IC 54 is made larger than the area of the bottom surface of the supporting base 12 in the conventional acceleration sensor shown in Fig. 2. That is, the hybrid IC 54 used for this type of application usually has an area of base larger than that of the supporting base 12 shown in Fig. 2. Consequently, the hybrid IC 54 can be secured to the base plate 52 more firmly, as compared with the supporting base 12 shown in Fig. 2. Accordingly, in the acceleration sensor 51 according to the invention, the hybrid IC 54 is more difficult to strip from the base plate 52, as compared with the supporting base 12 shown in Fig. 2, thereby to effectively prevent such a failure that the piezoelectric element and a member supporting the piezoelectric element are stripped from the base plate. Furthermore, in the construction shown in Figs. 10 and 11, the piezoelectric element 31 is secured to the upper surface of the hybrid IC 54 in a cantilevered shape. Consequently, when a large mechanical shock is applied, the hybrid IC 54 is not stripped from the base plate 52 but the piezoelectric element 31 is stripped from the upper surface of the hybrid IC 54 before the stripping. As a result, the electrical connection between the electrode 31a on the lower surface of the piezoelectric element 31 and the electrode 54f on the hybrid IC 54 shown in Fig. 10 is released, thereby to break an electric circuit. Consequently, when such a fault occurs, the occurrence of the fault is immediately detected by the operation of a fault self-diagnostic circuit incorporated in the hybrid IC 54. Although in Figs. 10 and 11, the piezoelectric element 31 is secured to the flat upper surface of the hybrid IC 54 in a cantilevered shape, the piezoelectric element 31 may be secured to the upper surface of the hybrid IC in such a shape that its both ends are supported thereon, as in the modified example shown in Fig. 7. In combination with the subject matter of claim 1, the acceleration sensor 51 according to the invention can also be characterized in that a temperature compensating capacitor is connected in parallel to the piezoelectric element. In an acceleration sensor using a piezoelectric element, an output voltage of the piezoelectric element is affected by the ambient temperature. Accordingly, temperature compensation has been conventionally made using a voltage amplifier. More specifically, in the conventional acceleration sensor, a piezoelectric element 61 is fixed on a supporting base 62 in a cantilevered shape within a case 60 represented by a broken line, as shown in Fig. 12. In addition, an impedance converter 63 is connected to the piezoelectric element 61, and a voltage amplifier 64 arranged outside the case 60 is connected to the impedance converter 63. In the voltage amplifier 64, the temperature compensation of the piezoelectric element 61 is made. However, the ambient temperature of the voltage amplifier 64 does not necessarily coincide with the ambient temperature of the piezoelectric element 61. Consequently, there arises the problem of making it impossible to make proper temperature compensation. On the other hand, the inventors of the present application have found that temperature compensation can be suitably made if a temperature compensating capacitor 72 is connected in parallel to a piezoelectric element 71 in close proximity to the piezoelectric element 71. In Fig. 13, reference numeral 73 denotes a supporting base. The supporting base 73 is the above described hybrid IC. In addition, reference numeral 74 denotes impedance converting means, and reference numeral 75 denotes a voltage amplifier. The impedance converting means 74 and the voltage amplifier 75 can be constructed as a single electronic component, for example, a hybrid IC within a case 70. Description is now made of the reason why temperature compensation can be suitably made when the temperature compensating capacitor 72 is connected in parallel to the piezoelectric element 71. Examination made by the inventors of the present application shows that the reason why an output voltage of the piezoelectric element is affected by the temperature is that the rate of variation of capacitance Q with temperature (ΔQs / Qs) and the rate of variation of piezoelectric stress constant d 31 with temperature (Δd 31 / d 31 ) differ from each other. In this case, Qs indicates the capacitance of the piezoelectric element, and d 31 indicates the piezoelectric stress constant of the piezoelectric element in the 31 direction. If the above described rate of variation of capacitance with temperature and the above described rate of variation of piezoelectric stress constant with temperature are equal to each other, the amount of change in output due to the change in piezoelectric stress constant is canceled by the amount of change in capacitance even if the temperature changes, thereby to make it possible to avoid the variation of the output voltage. More specifically, when the output voltage of the piezoelectric element is taken as V, the rate of variation of output voltage in a case where the output voltage V is changed by ΔV due to the rise in unit temperature is ΔV / V. "The output voltage V is not affected by the change in temperature" means that the above described rate of variation of output voltage becomes zero (ΔV / V = 0). If the rate of variation of output voltage (ΔV / V) is found, therefore, the output voltage V of the piezoelectric element is proportional to a stress (αG) produced by acceleration G at that time and the piezoelectric stress constant d 31 of the piezoelectric element, and is inversely proportional to the capacitance Qs of the piezoelectric element. That is, the following equation holds: V = αG · d 31 / Qs On the other hand, the amount of change in output voltage ΔV is as follows: ΔV = (V + ΔV) - V Accordingly, the following equation holds: ΔV = αG (d 31 + Δd 31 ) / (Qs + ΔQs) - αGd 31 / Qs Δd 31 in the foregoing equation (4) indicates the amount of change per unit temperature in piezoelectric stress constant. If the above described equations (2) and (4) are substituted in respective terms in the rate of variation of output voltage (ΔV / V) to eliminate common terms, the following equation holds: ΔV / V = [1 + (Δd 31 / d 31 )] / [1 + (ΔQs / Qs)] - 1 In order that the rate of variation of output voltage becomes zero in the above described equation (5), that is, ΔV / V = 0, the following equation may hold: ΔQs / Qs = Δd 31 / d 31 That is, it is found that the rate of variation of capacitance of the piezoelectric element and the rate of variation of piezoelectric stress constant thereof may be equal to each other. In the temperature characteristics of the piezoelectric element, however, the rate of variation of piezoelectric stress constant d 31 (Δd 31 / d 31 ) and the rate of variation of capacitance Qs (ΔQs / Qs) actually differ from each other, as shown in Fig. 14. Accordingly, an output voltage which is not affected by the change in temperature is not obtained. Therefore, the inventors of the present application has found that if the above described temperature compensating capacitor 72 is connected in parallel to the piezoelectric element 71 in close proximity to the piezoelectric element 71, temperature compensation can be reliably made by selecting the capacitance of the capacitor 72. More specifically, if the rate of variation of capacitance of the piezoelectric element (ΔQs / Qs) and the rate of variation of piezoelectric stress constant thereof (Δd 31 / d 31 ) are equal to each other as shown in the equation (6), the rate of variation of output voltage of the piezoelectric element becomes zero (ΔV / V = 0). If the capacitance Qt at the reference temperature of the capacitor 72 and the rate of variation of capacitance with temperature thereof (ΔQt / Qt) are suitably selected, the rate of variation of piezoelectric stress constant of a parallel circuit and the rate of variation of capacitance thereof are approximately equal to each other, thereby to obtain an output voltage which is not affected by the ambient temperature. In the above described capacitor 72, the required capacitance Qt can be easily calculated as follows. That is, if the condition under which the rate of variation of output voltage becomes zero is found by the foregoing equation (6), the following equation holds because the total capacitance of a parallel circuit of the piezoelectric element 71 and the capacitor 72 is the sum of the capacitance of the piezoelectric element 71 and the capacitance of the capacitor 72: (ΔQs + ΔQt) / (Qs + Qt) = Δd 31 / d 31 If the equation (7) is changed for simplification, the following equation is obtained: Qt / Qs = (ΔQs / Qs - Δd 31 / d 31 ) / (Δd 31 / d 31 - ΔQt / Qt) The capacitance Qt of the capacitor 72 and the rate of variation of capacitance thereof (ΔQt / Qt) are unknown out of the respective terms in the equation (8). However, the rate of variation of capacitance of the capacitor 72 is determined depending on which type of capacitor is the capacitor 72. On the other hand, the other terms are all known. Therefore, letting the capacitance Qs (at 20°C) of the piezoelectric element 71 = 900 pF, the rate of variation of capacitance thereof (ΔQs / Qs) = 0.0036, and the rate of variation of piezoelectric stress constant thereof (Δd 31 / d 31 ) = 0.0023, the required capacitance Qt of each of capacitors exhibiting the following characteristics UJ, RH and CG: (1) In the capacitor having the characteristic UJ, the rate of variation of capacitance (ΔQt / Qt) is set to -0.00075, so that the required capacitance Qt of the capacitor 72 becomes 383.6 pF. (2) In the capacitor having -the characteristic RH, the rate of variation of capacitance (ΔQt / Qt) is set to -0.00022, so that the required capacitance of the capacitor 72 becomes 464.3 pF. (3) In the capacitor having the characteristics CG, the rate of variation of capacitance (ΔQt / Qt) is set to approximately zero, so that the required capacitance Qt of the capacitor 72 becomes 508.7 pF. As described above, the type of capacitor 72 used is suitably selected to find the value of the rate of variation of capacitance of the capacitor 72, and this value and the other known values are substituted in the above described equation (8) to find the required capacitance Qt of the capacitor 72. Consequently, the rate of variation of piezoelectric stress constant and the rate of variation of capacitance of the parallel circuit of the piezoelectric element and the capacitor can be made approximately equal to each other by only selecting the capacitance of the capacitor 72 connected in parallel to the piezoelectric element 71 in the above described manner. Consequently, even if the ambient temperature changes, the amount of change in output due to the change in piezoelectric stress constant is canceled by the amount of change in capacitance. As a result, an output voltage which is not affected by the ambient temperature is obtained. Since the above described capacitor 72 is provided in close proximity to the piezoelectric element 71, and the capacitor 72 is always operated at approximately the same temperature as the piezoelectric element 71, therefore, it is possible to accurately make temperature compensation. Referring now to Figs. 15 to 19, description is made of a modified example of the present invention in combination with the subject matter of claim 1. This example is characterized in that in an acceleration sensor so constructed that an electronic component containing a signal processing circuit and the like is arranged on a base plate, a cushioning member for absorbing a thermal shock produced between the base plate and the above described electronic component due to the difference in coefficient of thermal expansion or modulus of elasticity is provided between the base plate and the electronic component. The cushioning member is also applicable to each of the acceleration sensors in the above described first to fourth embodiments. In this modified example, the thermal shock produced due to the difference in coefficient of thermal expansion or modulus of elasticity is absorbed by the above described cushioning member, so that an excessive thermal stress is not exerted on the electronic component for constructing signal processing means and the like. Consequently, the possibility of causing damages to the electronic component due to the thermal shock is prevented, thereby to enhance the reliability of the acceleration sensor. Furthermore, the thermal shock is absorbed by the above described cushioning member. Accordingly, even if a wiring pattern or a resistor film is formed on the bottom surface of the above described electronic component, it is hardly damaged. Accordingly, it is possible to also form a wiring pattern or a resistor film superior in reliability on the bottom surface of the electronic component, thereby to make it possible to make the electronic component used smaller in size and higher in integration density. Referring to Fig. 15, the acceleration sensor comprises a piezoelectric element 81 outputting an electric signal corresponding to acceleration and a hybrid IC 82 serving as an electronic component containing various types of signal processing circuits for processing a signal outputted from the piezoelectric element 81. The piezoelectric element 81 has a strip shape, and the hybrid IC 82 has a rectangular plate shape. The piezoelectric element 81 is held in the center of the hybrid IC 82 in a cantilevered shape. That is, a through hole 83 for mounting the piezoelectric element is formed in the center of the hybrid IC 82. A fixed end 81a of the piezoelectric element 81 is secured to an edge of the above described through hole 83, and a free end 81b thereof is so arranged as to face the through hole 83. A tip-type electronic component 84 and a wiring pattern (not shown) are secured or-formed on the upper surface of the hybrid IC 82. A component such as a resistor film or a wiring pattern, an electrode and the like are also formed on the bottom surface of the hybrid IC 82, which are not shown. The hybrid IC 82 and the piezoelectric element 81 are connected to each other through a bonding wire 86. In addition, the upper surface of the hybrid IC 82 is covered with a cover body 87 made of a metal, a conductive resin or a material obtained by metal-plating a synthetic resin. Meanwhile, if the cover body 87 and the wiring pattern (not shown) formed on the hybrid IC 82 are connected to each other by bonding of the cover body 87, it is possible to further enhance the reliability of the electrical connection. A plurality of connecting terminals for connection to outer portions 88 are attached to one end of the hybrid IC 82, and are projected sideward from the hybrid IC 82. The acceleration sensor according to the present embodiment comprises a base plate 89 made of a metal on which the above described hybrid IC 82 is mounted and an insulating resin film 90. This insulating resin film 90 serves as a cushioning member in the present invention. The base plate 89 comprises a mounting stage 89a for mounting the hybrid IC 82 in its central part, and the insulating resin film 90 is affixed on the mounting stage 89a with, for example, epoxy adhesives. A polymer film such as a polyimide film having a coefficient of thermal expansion of 8 × 10 -6 to 17 × 10 -6 /°C and having modulus of elasticity of 380 kg/mm 2 and a polyethylene terephthalate film having a coefficient of thermal expansion of 30 × 10 -6 to 50 × 10 -6 /°C and having modulus of elasticity of 400 kg/mm 2 are suitable as the insulating resin film 90, and approximately 10 µ m is sufficient and particularly, approximately 30 µ m is suitable for the thickness thereof. The hybrid IC 82 is affixed on the insulating resin film 90 affixed to the mounting stage 89a through, for example, epoxy adhesives. In order to firmly bond the base plate 89, the insulating resin film 90, and the hybrid IC 82, it is preferable that both surfaces of the insulating resin film 90 are previously subjected to sandblasting. The area above the hybrid IC 82 positioned and fixed on the base plate 89 is covered with a cap 91. The cap 91 is made of a metal or a conductive resin, and has dimensions covering the mounting stage 89a of the base plate 89. In addition, parts on the side of the bottom surface and one side surface of the cap 91 are opened. The lower parts of the side surfaces of the cap 91 are secured to the periphery of the mounting stage 89a so that the cap 91 is fixed on the base plate 89. Consequently, the hybrid IC 82 is covered with the cap 91 in a state where the connecting terminals for connection to outer portions 88 are projected sideward from the opened part of the cap 91. In addition, the cap 91 is filled with an insulating resin 92 such as silicone resin for the purpose of hermetical sealing. Since the piezoelectric element 81 is surrounded by the cover body 87, however, a space for vibration A of the piezoelectric element 81 is ensured. Also in this acceleration sensor, the hybrid IC 82 and the base plate 89 differ from each other in coefficient of thermal expansion and modulus of elasticity (the coefficient of thermal expansion of the hybrid IC 82 is 2 × 10 -6 to 7 × 10 -6 /°C and the modulus of elasticity thereof is 1 × 10 4 to 3 × 10 4 kg/mm 2 , while the coefficient of thermal expansion of the base plate made of metal is 10 × 10 -6 to 30 × 10 -6 /°C and the modulus of elasticity thereof is 0.1 × 10 4 to 2 × 10 4 kg/mm 2 ), as in the conventional acceleration sensor. Consequently, when the temperature violently changes, a thermal shock can be produced between the hybrid IC 82 and the base plate 89. However, the above described insulating resin film 90 is interposed therebetween, which functions as a cushioning member to absorb the above described thermal shock. Consequently, there occur no damages such as cracking or chipping and stripping in the hybrid IC 82. Furthermore, the piezoelectric element 81 is mounted on the hybrid IC 82. The hybrid IC 82 is reliably fixed on the base plate 89 without being stripped by the function of the insulating resin film 90. Consequently, a failure making it impossible to measure acceleration, that is, the stripping of the piezoelectric element 81 does not easily occur. Additionally, the thermal shock is absorbed by the insulating resin film 90, so that there is no possibility of damaging the wiring pattern or the resistor film formed on the bottom surface of the hybrid IC 82. Similarly, the surface of the wiring pattern or the resistor film is physically protected by the insulating resin film 90. Moreover, the electrical insulation between the bottom surface of the hybrid IC 82 and the base plate 89 is ensured. Accordingly, it is possible to form a wiring pattern or a resistor film superior in reliability on the bottom surface of the hybrid IC 82, thereby to make it possible to make the hybrid IC 82 smaller in size and higher in integration density. Although, as above described, the insulating resin film 90 is used as a cushioning member, the present invention is not limited to the same. For example, insulating adhesives used for bonding can be also used as the above described cushioning member. In this case, the insulating adhesives must be so applied as to have a thickness of at least approximately 10 µ m in order to obtain the cushioning effect. Furthermore, the connecting terminals for connection to outer portions 88 are directly mounted on the hybrid IC 82, and are not mounted on the base plate 89. Consequently, it is possible to omit complicated work such as work of mounting the connecting terminals for connection to outer portions 88 so as to pass through the base plate 89 or work of insulating the connecting terminals for connection to outer portions 88 and the base plate 89 from each other. Additionally, the above described bonding wire 86 need not be necessarily used so as to connect the piezoelectric element 81 and the hybrid IC 82 to each other. For example, the piezoelectric element 81 and the hybrid IC 82 may be connected to each other using solder, conductive adhesives or the like. Furthermore, as the piezoelectric element 81, a piezoelectric element of a bimorph type, a piezoelectric element of a unimorph type, and a piezoelectric element of a shear mode type (shearing type) may be used. In the piezoelectric element 81 of a shear mode type, the piezoelectric element 81 and the hybrid IC 82 can be connected to each other without using the bonding wire 86 from a structural point of view. Although in the example shown in Fig. 15, the piezoelectric element 81 is fixed to the upper surface of the hybrid IC 82 in a cantilevered shape, the piezoelectric element 81 may also be fixed to a flat upper surface of a hybrid IC 82 so as to support both ends of the piezoelectric element 81 on both sides by edges opposed to each other of a through hole 83 in the hybrid IC 82, as shown in Fig. 19. In this case, if grooves for positioning the piezoelectric element 81 and mounting the same are provided at both the edges of the through hole 83, the piezoelectric element 81 is easily positioned, and the necessity of making connection by a bonding wire is eliminated by only altering wiring on the hybrid IC 82, thereby to make it possible to further enhance reliability and productivity.

Claims: An acceleration sensor (51) comprising: an acceleration detecting portion (32) comprising a piezoelectric element (31) having electrodes (31a; 31b) on both its surfaces and outputting a signal corresponding to acceleration applied and a supporting base (33) for supporting said piezoelectric element (31), said acceleration detecting portion (32) being provided with a pair of detecting portions (34a; 34b) arranged so as to be brought into contact with said piezoelectric element (31) and spaced apart from each other by a predetermined distance; signal processing means (36) electrically connected to one of the electrodes (31b) of said piezoelectric element (31) and for processing the electric signal outputted from the piezoelectric element (31); and signal outputting means (35) electrically connected to one (34a) of said pair of detecting portions (34a; 34b) so as to cause a predetermined current to flow between said detecting portions and connected to said signal processing means (36) so as to output to the signal processing means (36) a state signal representing the state of the acceleration detecting portion (32), said signal outputting means (35) outputting said state signal in conformity with the variation of the current flowing between the detecting portions (34a; 34b). The acceleration sensor according to claim 1, wherein said pair of detecting portions (34a; 34b) is provided between said piezoelectric element (31) and the supporting base (33). The acceleration sensor according to claim 2, wherein said supporting base (33) is constituted by a pair of supporting members (33a; 33b) arranged spaced apart from each other by a predetermined distance, an electrode constituting said pair of detecting portions (34a; 34b) being formed on the upper surface of said pair of supporting members (33a; 33b). The acceleration sensor according to claim 3, wherein said pair of supporting members (33a; 34b) is arranged so as to support the piezoelectric element (31) in both ends of the piezoelectric element (31) or in the vicinity thereof. The acceleration sensor according to claim 2, wherein said supporting base (33) is arranged so as to support the piezoelectric element (31) in one end of the piezoelectric element (31) or in the vicinity thereof, a pair of electrodes constituting said pair of detecting portions (34a; 34b) arranged spaced apart from each other by a predetermined distance being formed on the upper surface of said supporting base (33). The acceleration sensor according to claim 1, wherein said signal processing means (36) comprises impedance converting means (37) electrically connected to the piezoelectric element (31) and for converting the impedance of the signal outputted from the piezoelectric element (31), filtering means (38) electrically connected to the impedance converting means (37) and for removing an unnecessary signal component, and amplifying means (39) electrically connected to said filtering means (38) and for amplifying a signal applied from the filtering means (38). The acceleration sensor according to claim 1, wherein at least a part of said signal processing means (36) is constituted by a single electronic component (54) having a flat upper surface, said piezoelectric element (31) being fixed to said upper surface of the electronic component (54) in a cantilevered shape. The acceleration sensor according to claim 1, which further comprises a temperature compensating capacitor (72) arranged in close proximity to said piezoelectric element (31) and connected in parallel to the piezoelectric element (31). The acceleration sensor according to claim 1, wherein at least a part of said signal processing means (36) is constituted by a single electronic component (54), and which further comprises a base plate (52) for supporting said electronic component (54), a cushioning member (90) for absorbin a thermal shock produced between the base plate (52) and said electronic component (54) due to the difference in coefficient of thermal expansion or modulus of elasticity being arranged between the base plate (52) and the electronic component (54).

Inventors: INOUE JIRO, MIZUNO TOSHIHIRO, TABOTA JUN, YAMASHITA MUNEHARU, INOUE, JIRO, MIZUNO, TOSHIHIRO, TABOTA, JUN, YAMASHITA, MUNEHARU, INOUE, JIRO, C/O MURATA MANUFACT. CO., LTD, MIZUNO, TOSHIHIRO, C/O MURATA MANUFACT. CO., LTD, TABOTA, JUN, C/O MURATA MANUFACT. CO., LTD, YAMASHITA, MUNEHARU, C/O MURATA MANUFACT. CO., LTD
Applicants: MURATA MANUFACTURING CO, MURATA MANUFACTURING CO., LTD.
Application year: 1992
Main classification: G01P15
IPC codes: G01P1, G01P15