Piezoelectric pressure sensor

Abstract: A piezoelectric pressure sensor is provided with a sensor housing including an inner sensor housing (12), accommodating an piezoelectric element (15), and an outer sensor housing (10) threaded to a test article. The inner sensor housing is attached to the outer sensor housing in such a manner that a prestress applied to the piezoelectric element does not change due to an axial strain in the outer sensor housing. The axial strain is produced when the outer sensor housing is mounted to a test article.

Description: The present invention relates to a piezoelectric pressure sensor and more particularly but not exclusively relates to a piezoelectric pressure sensor suitable for detecting combustion pressure within a cylinder of an internal combustion engine, for example. Piezoelectric pressure sensors utilising piezoelectric effect in which electric charges are generated by applying stress have been widely used. Recently, a piezoelectric pressure sensor suitable for detecting combustion pressure within a cylinder of an internal combustion engine has been intensively developed. Fig. 3 is an axial section of a piezoelectric pressure sensor disclosed in Japanese Patent Unexamined Publication No. 63-109342. The piezoelectric pressure sensor is of the type in which a shearing stress is applied to a piezoelectric element 24 thereof. The piezoelectric element 24 is arranged within a sensor housing 21. The piezoelectric element 24 is secured and prestressed in such a manner that the piezoelectric element 24 is urged by a fastening screw member 25 against a pressure receiving surface 22 through a pressure transmitting member 23. This pressure is necessary for measuring negative pressure particularly in measuring combustion pressure within a cylinder of an internal combustion engine. The pressure applied to the pressure receiving surface 22 is transmitted to an inner circumferential portion of the piezoelectric element 24 through the pressure transmitting member 23 which is electrically insulated from the sensor housing 21. In this case, a shearing stress is produced in the piezoelectric element 24 since the piezoelectric element 24 is held by an upper outer circumferential portion thereof by the upper fastening screw member 25. Thus, electric charges are generated in the inner and outer circumferential surfaces of the piezoelectric element 24 in accordance with the shearing stress, and the electric charges are detected as an electric signal from electrodes provided on the inner and outer circumferential surfaces. The piezoelectric pressure sensor with such a construction is advantageous in that it has a simple structure. However, when the piezoelectric pressure sensor is threaded to a test article 32, tensile stresses are produced in the flange of the sensor housing 21 in proportion to a tightening force of the sensor housing 21 to the test article 32 with tensile stresses generated as shown in Fig. 4, so that the distance between the pressure receiving surface 22 and the upper fastening screw 25 is enlarged. As a result, the prestress applied to secure or fix the piezoelectric element 24 is reduced, and hence contacts between the pressure transmitting member 23 and the piezoelectric element 24 and between the piezoelectric element 24 and the fastening screw member 25 become unstable. This causes pressure on the pressure receiving surface 22 to be nonuniformly transmitted or partially transmitted to the piezoelectric element 24, resulting in a considerable fluctuation in the senor output. Moreover, the sensor output is adversely affected since the piezoelectric constant of the piezoelectric element changes in accordance with the magnitude of the prestressing. Accordingly, it is an object of the present invention to provide a piezoelectric pressure sensor which is capable of generating no change in the prestressing which secures the piezoelectric element, thereby producing a stable output. In view of this and other objects, the present invention provides a piezoelectric element; an inner sensor housing which houses the piezoelectric element, said inner sensor housing including a pressure receiving wall, pressure transmitting means for transmitting pressure to be measured applied to an outer surface of said wall to the piezoelectric element, and means for urging the piezoelectric element against the pressure transmitting means to apply an axial prestress to said element, characterised in that there is provided an outer sensor housing having means to enable the housing to be screwed into a test article in which pressure is to be measured, the outer sensor housing houses the inner sensor housing such that the pressure receiving wall of the inner sensor housing is subjected to the pressure to be measured in the test article; and means for securing the inner sensor housing within the outer sensor housing. Fig. 1 is an axial cross-sectional view of a piezoelectric pressure sensor according to an embodiment of the present invention; Fig. 2 is an axial cross-sectional view of a modified form of the piezoelectric pressure sensor of Fig. 1; Fig. 3 is an axial cross-sectional view of a typical example of a conventional piezoelectric pressure sensor; and Fig. 4 is an axial cross-sectional view of the conventional piezoelectric pressure sensor mounted to a test article and illustrates a state in which tensile stresses are applied to the piezoelectric pressure sensor. Referring to Fig. 1, a piezoelectric pressure sensor according to an embodiment of the present invention will be described. The piezoelectric pressure sensor includes a substantially hollow cylindrical, outer sensor housing 10 and an inner sensor housing 12. The inner sensor housing 12 is fitted in the outer sensor housing 10. The outer housing 10 includes a screwing portion 110, an intermediate portion 111 having larger outer diameter than the screwing portion 110 and a largest diameter portion 112. The screwing portion 110, the intermediate portion 111 and the largest diameter portion 112 are formed integrally and coaxially. Between the screwing portion 110 and intermediate portion 111, an outer annular shoulder 113 is formed in an outer surface of the outer sensor housing 10 while an inner annular shoulder 114 is formed in an inner surface of the outer sensor housing 10. The screwing portion 110 is provided with a screw thread 115 formed in the outer surface thereof. The inner sensor housing 12 has a sensing portion 120 and a proximal portion 121 coaxially and integrally formed with the sensing portion 120. An outer annular shoulder 122 is formed in an outer surface of the inner sensor housing 12 between the sensing portion 120 and proximal portion 121. The proximal portion 121 is provided in an outer surface thereof with a male screw thread 123, which tightly engages with a female screw thread 116 formed in an inner surface of the intermediate portion 111 so that an annular sealing member 17 is clamped between the inner annular shoulder 114 of the outer sensor housing 10 and the outer annular shoulder 122 of the inner sensor housing 12 for hermetical sealing. The sensing portion 120 is slidably fitted into the screwing portion 110 of the outer sensor housing 11 and is not fixed to the screwing portion 110. Thus, the sensing portion 120 is capable of axially moving without any change in axial length when axial strains are produced in the outer sensor housing 10, particularly in the intermediate portion 111 when mounting to a test article. A pressure receiving bottom wall 13 is provided on a tip of the inner sensor housing 12 to be exposed at an outer surface 130 thereof to pressure to be measured. An elongated pressure transmitting member 14 is arranged to contact at one end thereof to an inner surface of the pressure receiving bottom wall 13 through a thin electrical insulation layer 131 such as Teflon (the trademark for polytetrafluoroethylene). The pressure transmitting member 14 may be made of an electrically insulating material such as structural ceramic. The pressure transmitting member 14 is provided at the other end thereof with a diameter reduced portion 140 coaxial with said end. A hollow cylindrical piezoelectric element 15 fits with a lower inner circumferential portion 151 around the diameter reduced portion 140 of the pressure transmitting member 14. The piezoelectric element 15 has a polarisation axis along the axis thereof and is respectively provided on its inner and outer circumferential surfaces with electrodes (not shown). A fastening screw member 16 fits around an upper outer circumferential portion 152 of the piezoelectric element 15. The fastening screw member 16 is threaded at a head portion 160 thereof to an inner circumferential surface 124 of the proximal portion 121 for urging the piezoelectric element 15 against the pressure receiving bottom wall 13 through the pressure transmitting member 14 to provide a prestress to the piezoelectric element 15. In this manner, the piezoelectric element 15 and the pressure transmitting member 14 are secured in the inner sensor housing 12. The fastening screw member 16 is electrically connected to the electrodes, provided on the outer circumferential surface of the piezoelectric element 15, and serves as lead wires. In this piezoelectric pressure sensor, pressure, applied to the pressure receiving bottom wall 13 due to variation in pressure in a zone to be measured, is transmitted to the lower inner circumferential portion 151 of the piezoelectric element 15 through the pressure transmitting member 14. The piezoelectric element 15 is held at the upper outer circumferential portion 152 by the fastening screw member 16, and hence the pressure transmitted to the piezoelectric element 15 causes a shearing stress to be generated in the piezoelectric element 15, so that, in proportion with the shearing stress, electric charges are generated in the electrodes of the piezoelectric element 15. Fig. 2 illustrates a modified form of the piezoelectric pressure sensor of Fig. 1. In the modified piezoelectric pressure sensor, the proximal portion 121 of the inner sensor housing 12 is welded at an upper edge portion 126 thereof to the inner circumferential surface of the intermediate portion 111 of the outer sensor housing 10. Instead of welding, the proximal portion 121 may be brazed at the upper edge portion 126 to the intermediate portion 111. Such welding or brazing of the proximal portion 121 produces a similar effect as the screw thread engagement in the preceding embodiment. The piezoelectric pressure sensor of the present invention does not produce any change in the prestress in the piezoelectric element 15 due to axial strains of the sensor housing as in the prior art and hence reduces fluctuation of the sensor output. The piezoelectric pressure sensor of this embodiment uses shearing stress to sense pressure and is hence advantageous in that the influence of a pyroelectric output of the piezoelectric element is reduced. Although the piezoelectric pressure sensor of a shear effect type using shearing stress is illustrated in the embodiment, it is possible to provide a piezoelectric pressure sensor utilizing stresses in the compression direction or the radial direction of the piezoelectric element. The piezoelectric element may be made of a material containing a major proportion of lead titanate added with Mn 15 and La. Lead titanate provides excellent thermal resistance and a stable characteristic at high temperatures for the piezoelectric element. The piezoelectric element has enhanced output characteristics at high temperature. When a piezoelectric element using shearing stress as in the present invention is made of such a lead titanate material, a piezoelectric pressure sensor suitable for sensing pressure at high temperatures is provided.

Claims: A piezoelectric pressure sensor of the type in which electric charges generated by applying a dynamic stress to a piezoelectric element are detected, comprising: a piezoelectric element (15); an inner sensor housing (12) which houses the piezoelectric element (15), said inner sensor housing (12) including a pressure receiving wall (13), pressure transmitting means (14) for transmitting pressure to be measured applied to an outer surface (130) of said wall (13) to the piezoelectric element (15), and means (16) for urging the piezoelectric element against the pressure transmitting means (14) to apply an axial prestress to said element, characterised in that there is provided an outer sensor housing (10) having means (115) to enable the housing (10) to be screwed into a test article in which pressure is to be measured, the outer sensor housing (10) houses the inner sensor housing (12) such that the pressure receiving wall (13) of the inner sensor housing (12) is subjected to the pressure to be measured in the test article; and means (116, 123, 126) for securing the inner sensor housing (12) within the outer sensor housing (10). A piezoelectric pressure sensor as claimed in claim 1, characterised in that the outer sensor housing (10) has a substantially cylindrical shape, including a fitting portion (110) adapted to fit into the test article, the fitting portion (110) having an outer surface; an intermediate portion (111) coaxial with the fitting portion (110) and having a larger outer diameter than that of the fitting portion (110), the intermediate portion (111) having an inner surface; an outer annular shoulder (113) integrally formed with both the fitting portion (110) and the intermediate portion (111); and the means for enabling the housing (10) to be screwed to the test article comprises a screw thread (115) formed in the outer surface of the fitting portion for engaging with the test article, the outer annular shoulder (113) being adapted to be urged against the test article to generate axial stress therein when the screw thread is engaged to the test article; the inner sensor housing (12) comprises a sensor (120) adapted to be arranged within the fitting portion (110) of the outer sensor housing in a manner to be axially movable relative to the latter; and has a proximal portion (121) coaxial with the sensor portion (120) and located on a side of the outer annular shoulder (113) of the outer sensor housing opposite to a side of the outer annular shoulder (113) on which said pressure receiving surface (130) is located; and the securing means (116, 123, 126) arranged between the proximal portion (121) of the inner sensor housing (12) and the inner surface of the intermediate portion (111) of the outer sensor housing (10) for securing the proximal portion of the inner sensor housing to the inner surface of the intermediate portion. A piezoelectric pressure sensor as claimed in claim 2, characterised in that the securing means (116, 123) comprises a male screw thread (123), formed in the proximal portion (121) of the inner sensor housing, and a female screw thread (116) formed in the inner surface of the intermediate portion of the outer sensor housing to engage the male screw (123). A piezoelectric pressure sensor as claimed in claim 3, characterised in that the proximal portion (121) of the inner sensor housing (12) is larger in outer diameter than the sensing portion (120) thereof to form an annular outer shoulder; the outer sensor housing comprises an inner shoulder adapted to engage with the outer shoulder (122) of the inner sensor housing; and further comprising a sealing member (17) adapted to be interposed between the inner shoulder (122) of the outer sensor housing and the outer shoulder (114) of the inner housing for hermetically sealing the inner sensor housing. A piezoelectric pressure sensor as claimed in claims 1 or 2, characterised in that the securing means (126) is a welded portion joining the proximal portion (121) of the inner sensor housing to the inner surface of the intermediate portion (111) of the outer sensor housing. A piezoelectric pressure sensor as claimed in claim 5, characterised in that the securing means (116) is a brazed portion joining the proximal portion (121) of the inner sensor housing to the inner surface of the intermediate portion (111) of the outer sensor housing. A piezoelectric pressure sensor as claimed in any preceding claim, characterised in that the pressure applying means (16) and the pressure transmitting means (14) are adapted to support the piezoelectric element (15) to produce a shearing stress in the piezoelectric element (15) when the pressure is transmitted from the pressure transmitting means (14) to the piezoelectric element (15), whereby electric charges generated due to the shearing stress are detected. A piezoelectric pressure sensor as claimed in claim 7, characterised in that the piezoelectric element (15) is generally in the shape of a hollow cylinder and includes an inner peripheral portion (151) and an outer peripheral portion (152); the piezoelectric element (11) is adapted to attach at the inner peripheral portion (151) thereof to the pressure transmitting means (14) and at the outer peripheral portion (152) thereof to the pressure applying means (16); the inner sensor housing (12) has an inner surface (124); and the pressure applying means (16) is threaded to the inner surface or the inner sensor housing for applying the prestress to the piezoelectric element (15). A piezoelectric pressure sensor as claimed in any preceding claim, characterised in that the piezoelectric element (15) is made of a material containing a lead titanate.

Inventors: KUSAKABE HIROKI, TAKIGAWA MASUO, KUSAKABE, HIROKI, TAKIGAWA, MASUO
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: G01L9
IPC codes: G01L9, G01L1