JP3993857B2 - Pressure sensor - Google Patents

Description

  The present invention relates to a pressure sensor, and more particularly to a pressure sensor that can be suitably used in a high temperature and high pressure environment such as in a cylinder of an internal combustion engine.

  As a sensor for detecting pressure in a high temperature and high pressure environment such as in a cylinder of an internal combustion engine, a pressure sensor using a piezoelectric element as a pressure detection unit is known. The piezoelectric element can be used continuously at a high temperature and has a high-speed response characteristic, so that it can be suitably used for pressure detection under a high temperature and high pressure environment. Conventionally, quartz, gallium phosphate, lithium niobate, langasite, and the like are known as piezoelectric elements used for pressure detection (see Patent Document 1 and Patent Document 2).

  In the pressure sensor shown in FIG. 2, a diaphragm 20 is sealed at a front end portion of a main body 10 formed in a cylindrical shape, and a piezoelectric sensor 22 is arranged inside a head portion 15 formed on the front end side of the main body 10. The charge signal generated in the piezoelectric sensor 22 due to the pressure applied to the diaphragm 20 is detected by a detector via the lead pin 40 and the receptacle 42. The illustrated pressure sensor uses a piezoelectric element utilizing the horizontal axis effect, and a plurality of such piezoelectric elements are stacked to form the piezoelectric sensor 22.

A pressure sensor that uses a piezoelectric element for the pressure detection unit is used in a state where the piezoelectric element is pressurized at a constant pressure. In the pressure sensor shown in FIG. 2, the front electrode 24 and the rear electrode 26 are arranged with the piezoelectric sensor 22 interposed therebetween, and the rear electrode is interposed by the first inner body 32 and the second inner body 34 via the insulating ring 28. A predetermined pressure acts on the piezoelectric sensor 22 by pressing 26. The front electrode 24 is formed integrally with the diaphragm 20, and the diaphragm 20 is attached to the front end portion of the main body 10 by welding.
JP-A-5-172680 JP-A-10-54773

As described above, the pressure sensor using a piezoelectric element for the pressure detection unit is assembled by adjusting so that a predetermined pressure acts on the piezoelectric sensor 22 when the pressure sensor is assembled. By assembling the piezoelectric sensor 22 in a state where a preload is applied, an output signal can be reliably extracted from the piezoelectric sensor 22.
However, when the pressure sensor is actually attached to an engine or the like and the pressure of the object to be measured is measured, when the pressure sensor is heated to a high temperature, the zero point position of the output signal from the pressure sensor fluctuates from the reference position. A problem arises.

As described above, when the zero point position of the output signal of the pressure sensor fluctuates, the output signal from the pressure sensor does not reflect the accurate pressure value of the object to be measured, and the reliability of the measurement data by the pressure sensor is impaired. Result.
Accordingly, the present invention has been made to solve these problems, and the object of the present invention is to provide a condition in which the pressure sensor is heated to a high temperature during use, as in the case of detecting the pressure in the engine. Even in this case, it is possible to provide a pressure sensor that suppresses fluctuations in the position of the zero point of the output signal from the pressure sensor, thereby enabling stable and highly reliable pressure detection.

In order to achieve the above object, the present invention comprises the following arrangement.
That is, in a pressure sensor comprising: an external housing incorporating a piezoelectric sensor; and an internal structure including the piezoelectric sensor that is disposed in the external housing and transmits pressure from a measurement object to the piezoelectric sensor. The internal structure includes a front electrode and a rear electrode arranged with a piezoelectric sensor interposed therebetween, and the thermal expansion coefficient of the piezoelectric sensor is smaller than the thermal expansion coefficient of the outer casing, and the thermal expansion coefficient of the rear electrode There the set larger than the thermal expansion coefficient of the outer housing, that the amount of thermal expansion between the inner structure and the outer housing is matched, to prevent fluctuations in the preload force acting on the piezoelectric sensor Features .

In addition, in the pressure sensor comprising: an external housing incorporating a piezoelectric sensor; and an internal structure including the piezoelectric sensor that is disposed in the external housing and transmits pressure from the measurement target to the piezoelectric sensor. A front electrode and a rear electrode arranged with a piezoelectric sensor sandwiched between the inner structure and a rear part of the rear electrode, and fixed to the outer casing with a preload applied to the piezoelectric sensor via an insulating ring A thermal expansion coefficient of the piezoelectric sensor and the insulating ring is smaller than a thermal expansion coefficient of the outer casing, and a thermal expansion coefficient of the rear electrode and the first inner body is The coefficient of thermal expansion of the external casing is set to be larger than that of the external casing, and the thermal expansion amounts of the external casing and the internal structure are matched.
In addition, a diaphragm is formed integrally with the front electrode at the front end of the outer casing and sealed, and the piezoelectric sensor is sandwiched between the front electrode and the rear electrode provided at the rear of the diaphragm. It is characterized by being arranged.
The piezoelectric sensor may be formed by laminating a plurality of piezoelectric elements made of a langasite crystal using a horizontal axis effect.

  The pressure sensor according to the present invention has a configuration in which the thermal expansion amounts of the external housing and the internal structure are matched, so that the temperature of the measured object rises and the temperature of the pressure sensor itself rises from room temperature. Even in this case, it is possible to prevent the pressure acting on the piezoelectric sensor from fluctuating from the pre-pressure set at the time of assembly, making it possible to accurately detect the pressure of the measured object, and a stable and reliable pressure sensor. It becomes possible to provide as.

Hereinafter, preferred embodiments of the present invention will be described in detail.
FIG. 1 is a cross-sectional view showing a configuration of an embodiment of a pressure sensor according to the present invention. The figure shows an enlarged configuration of the head portion 15 of the pressure sensor in which the piezoelectric element is built. The configuration of the head unit 15 is the same as the configuration of the pressure sensor shown in FIG. 2 described above, and the front electrode 24 and the rear electrode 26 are arranged with the piezoelectric sensor 22 in between, and the insulating ring 28 and the rear electrode 26 are interposed therebetween. A predetermined preload is applied to the piezoelectric sensor 22 by the first inner body 32.

  As shown in FIG. 2, the first inner body 32 contacts the rear surface of the insulating ring 28, and the second inner body 34 contacts the rear surface of the first inner body 32. A screw is provided on the outer peripheral surface of the second inner body 34, and the second inner body 34 is screwed with a screw provided on the inner peripheral surface of the main body 10. The preload applied to the piezoelectric sensor 22 can be appropriately set by adjusting the position of the second inner body 34 in the axial direction during assembly. After adjusting the preload, laser welding is performed between the first inner body 32 and the main body 10, and the first inner body 32 is fixed to the main body 10 so that a predetermined pressure is applied to the piezoelectric sensor 22. Retained. Reference numeral 36 denotes a laser welded portion.

  In the conventional pressure sensor shown in FIG. 2, the main body 10, the diaphragm 20, the rear electrode 26, and the first inner body 32 are all made of the same stainless steel material. The piezoelectric sensor 22 is a piezoelectric element made of a langasite crystal, and the insulating ring 28 is alumina ceramic.

For example, when the pressure sensor is attached to the engine to detect the pressure in the engine, the diaphragm 20 is directed into the engine room and the pressure sensor is fixed to the engine body. In FIG. 1, reference numeral 11 denotes a screw portion provided on the outer peripheral surface of the head portion 15 of the main body 10, and the pressure sensor of the present embodiment is formed so that the head portion 15 is screwed and fixed to a body to be detected. .
Therefore, when the pressure in the engine is actually detected, not only the diaphragm 20 and the head unit 15 but also the piezoelectric sensor 22 incorporated in the head unit 15, the rear electrode 26, the insulating ring 28, the first inner The body 32 and the like are heated to a high temperature.

  FIG. 6 shows data obtained by measuring the combustion pressure of an automobile engine using a conventional pressure sensor. As shown in the figure, the output signal from the pressure sensor indicates that a spike-like output signal appears repeatedly in response to repeated combustion in the engine room. At the same time, FIG. 6 shows that the zero point position gradually drifts downward as time elapses from the start of combustion of the engine.

The present inventors investigated the cause of the fluctuation of the 0-point position of the output signal of the pressure sensor in this way. The cause of the fluctuation of the 0-point position is that the main body 10 which is an external housing of the pressure sensor and the head It was found that the thermal expansion coefficients of the internal structures such as the piezoelectric sensor 22, the front electrode 24, the rear electrode 26 and the like built in the part 15 are different.
That is, the amount of thermal expansion between the external housing and the internal structure is different at room temperature when the pressure sensor is assembled and when the pressure sensor is heated to a high temperature, and acts on the piezoelectric sensor 22 due to the difference in the amount of thermal expansion. It was found that the pressure (pre-pressure) fluctuated and the zero point position of the output signal of the pressure sensor fluctuated.

  The internal structure is a member arranged inside a cylindrical outer casing, and refers to a member or a part of a member that takes into account the amount of thermal expansion in relation to the outer casing. In the present embodiment shown in FIG. 1, the internal structure for which the thermal expansion amount is compared and examined in relation to the external housing is from the laser welded portion 36 obtained by laser welding the main body 10 and the first inner body to the main body 10. Members (range L in the figure) arranged in the front portion, that is, the front electrode 24, the piezoelectric sensor 22, the rear electrode 26, the insulating ring 28, and the first inner body 32 formed integrally with the diaphragm 20. It will be a part of.

In the conventional pressure sensor, the thermal expansion coefficient of the stainless steel (SUS # 630) forming the main body 10, the diaphragm 20, the rear electrode 26, and the first inner body 32 is 11 × 10 ⁻⁶ (/ ° C.). The thermal expansion coefficient of the piezoelectric sensor 22 made of langasite crystal is 5-6 × 10 ⁻⁶ (/ ° C.). The thermal expansion coefficient of the alumina ceramic constituting the insulating ring 28 is 7 to 8 × 10 ⁻⁶ (/ ° C.). Comparing these thermal expansion coefficients, it can be seen that the thermal expansion coefficient of the piezoelectric sensor 22 is considerably smaller than the thermal expansion coefficient of other stainless steel members.

In the case of the pressure sensor shown in FIG. 1, due to its structure, the amount of thermal expansion of the external housing composed of the main body 10 and the internal structure incorporated in the head unit 15 can be calculated and obtained separately.
That is, in the range L shown in FIG. 1, by comparing the thermal expansion amount of the main body 10 with the thermal expansion amount of the internal structure built in the head portion 15, the temperature of the external housing and the internal structure as well as the temperature are compared. It can be determined by calculating how the amount of thermal expansion changes.

FIG. 4 shows the results of calculating the expansion amount of the external housing and the expansion amount of the internal structure. In this calculation, it is assumed that the main body 10, the diaphragm 20, the rear electrode 26, and the first inner body 32 are made of the same stainless steel (SUS # 630), and the thermal expansion coefficient of the piezoelectric sensor 22 is 6 × 10 ^−6. The thermal expansion coefficient of the insulating ring 28 was determined as 7 × 10 ⁻⁶ (/ ° C.). The amount of thermal expansion varies depending on the length of the head portion 15, the thickness (length) of the piezoelectric sensor 22, the thickness (length) of the rear electrode 26, and the like. The illustrated example is based on the design values shown in FIG.
FIG. 4 shows that when the thermal expansion coefficient of the outer casing and the thermal expansion coefficient of the internal structure are different, the difference in thermal expansion amount gradually increases with temperature.

  Thus, if the thermal expansion coefficient of the external housing and the thermal expansion coefficient of the internal structure are different, the difference in thermal expansion amount with the temperature increases, resulting in the pressure acting on the piezoelectric sensor 22 (pre-pressure). ) Fluctuates (in this embodiment, the pressure acting on the piezoelectric sensor 22 is reduced). As a result, as shown in FIG. 6, it is considered that the zero point position of the output signal of the pressure sensor gradually drifts as the temperature of the pressure sensor rises.

  The pressure sensor of the present embodiment is that the material of the internal structure is selected so that the difference in thermal expansion between the external housing and the internal structure does not occur even when the pressure sensor becomes high temperature. Features. That is, the piezoelectric sensor 22 constituting the internal structure has a smaller coefficient of thermal expansion than the outer casing made of stainless steel, and the amount of thermal expansion when the temperature rises is smaller than that of the outer casing. The amount of thermal expansion is compensated by the amount of thermal expansion of the other members constituting the internal structure, thereby matching the amount of thermal expansion of the entire internal structure with the amount of thermal expansion of the external housing.

In the present embodiment, the same stainless material as that of the main body 10 is actually used for the diaphragm 20, and different stainless steel materials are used for the rear electrode 26 and the first inner body 32. Specifically, the stainless steel (SUS # 303 or # 304) has a higher thermal expansion coefficient than the stainless steel (thermal expansion coefficient 11 × 10 ⁻⁶ (/ ° C.)) constituting the external housing. An expansion coefficient of 17 × 10 ⁻⁶ (/ ° C.) was used for the rear electrode 26 and the first inner body 32.

FIG. 3 shows how the amount of thermal expansion of the outer casing and the inner structure changes with temperature when the other electrode stainless steel is used for the rear electrode 26 and the first inner body 32. The results obtained are shown below. The shape and arrangement of internal structures such as the piezoelectric sensor 22 and the rear electrode 26 are the same as those shown in FIG.
The graph shown in FIG. 3 shows that when the above materials are used as the rear electrode 26 and the first inner body 32, the thermal expansion amounts of the outer casing and the inner structure are matched, and the temperature rises. This also shows that there is no difference in thermal expansion.

  FIG. 5 shows a case where a pressure sensor in which the thermal expansion amounts of the outer casing and the internal structure are matched is attached to an automobile engine, and the combustion pressure is measured under the same conditions as when the conventional pressure sensor shown in FIG. 6 is used. The results are shown. Also in FIG. 5, as in the case of using the conventional pressure sensor, spike-like output signals appear repeatedly with the combustion of the engine. A characteristic point in the measurement data shown in FIG. 5 is that there is almost no variation of the zero point position of the measurement data. The fluctuation of the zero point position is clear when compared with the measurement data using the conventional pressure sensor shown in FIG.

In FIG. 6, there is a tendency that the 0 point position gradually decreases from the start of combustion. This corresponds to the fact that the temperature of the pressure sensor itself gradually increases with the start of combustion of the engine, and the difference in thermal expansion between the external housing and the internal structure gradually increases. On the other hand, in the measurement data of the pressure sensor of the present embodiment shown in FIG. 5, the amount by which the zero point position varies with the passage of time from the start of combustion is small. This is because the thermal expansion amounts of the outer casing and the inner structure are matched.
As can be seen by comparing FIGS. 5 and 6, according to the pressure sensor of the present embodiment, it is possible to accurately measure the pressure value of the object to be measured by suppressing the fluctuation amount of the zero point position to be small. Therefore, it can be used as a stable and reliable pressure sensor.

  In the above embodiment, the material of the rear electrode 26 and the first inner body 32 in the internal structure is different from the material of the main body 10 as the external housing. The method of matching the amount of thermal expansion of is not limited to the above method. For example, the material of other members constituting the internal structure can be changed. Further, since the amount of thermal expansion is also related to the physical dimensions of each member, it can be adjusted by changing the design (thickness, length) of the member.

  Further, instead of changing the material of the internal structure, it is possible to achieve matching by changing the material of the main body 10 which is an external housing. Since the difference in thermal expansion between the external housing and the internal structure is relative, it can be designed by comparing and examining the thermal expansion of either or both of the external housing and the internal structure. is there. Moreover, even if it is a stainless steel material as in the above-described embodiment, a material having a different thermal expansion coefficient is used, and a material having a different thermal expansion coefficient is combined with the external housing. It is also possible to adjust the thermal expansion amount with the internal structure to match.

  The present invention is not limited to the pressure sensor according to the configuration described in the above embodiment, and the pressure sensor is configured to include an internal structure including a piezoelectric sensor in an external housing and extract a signal from the piezoelectric sensor. Can be applied in common. In this case, the pressure sensor is not limited to a product having a structure in which a constant pressure is always applied to the piezoelectric sensor. The present invention can be similarly applied to a pressure sensor having a structure that can reliably extract a charge signal without applying a preload to the piezoelectric sensor using a conductive bonding agent or the like. Moreover, in the said embodiment, although it is set as the piezoelectric sensor 22 using the piezoelectric element using the horizontal axis effect which consists of a langasite crystal, it can apply similarly also to the pressure sensor using another piezoelectric element. Further, the present invention can be similarly applied to a pressure sensor configured using another piezoelectric element in a form different from that of the above embodiment.

It is sectional drawing which expands and shows the structure of the head part of the pressure sensor which concerns on this invention. It is sectional drawing which shows the whole structure of a pressure sensor. It is a graph which shows the temperature change of the amount of thermal expansion in the state where the amount of thermal expansion of an external case and an internal structure was matched. It is a graph which shows a mode that the amount of thermal expansion of an external housing and an internal structure changes with temperature. It is a graph which shows the result of having measured pressure using the pressure sensor concerning the present invention. It is a graph which shows the result of having measured pressure using the conventional pressure sensor.

Explanation of symbols

DESCRIPTION OF SYMBOLS 10 Main body 15 Head part 20 Diaphragm 22 Piezoelectric sensor 24 Front electrode 26 Rear electrode 28 Insulation ring 30 Insulation sleeve 32 1st inner body 34 2nd inner body 36 Laser welding part 40 Lead pin

Claims (4)

In a pressure sensor comprising: an external housing incorporating a piezoelectric sensor; and an internal structure including the piezoelectric sensor, which is disposed in the external housing and transmits pressure from a measurement object to the piezoelectric sensor.
The internal structure includes a front electrode and a rear electrode arranged with a piezoelectric sensor interposed therebetween,
A thermal expansion coefficient of the piezoelectric sensor is smaller than a thermal expansion coefficient of the outer casing;
The thermal expansion coefficient of the external housing is set to be larger than that of the external housing , and the amount of thermal expansion between the external housing and the internal structure is matched, so that the fluctuation of the pre-pressure acting on the piezoelectric sensor is reduced. A pressure sensor characterized by prevention. In a pressure sensor comprising: an external housing incorporating a piezoelectric sensor; and an internal structure including the piezoelectric sensor, which is disposed in the external housing and transmits pressure from a measurement object to the piezoelectric sensor.
A front electrode and a rear electrode disposed with the internal structure sandwiching the piezoelectric sensor, and a rear portion of the rear electrode, and a preload is applied to the piezoelectric sensor via an insulating ring. A fixed first inner body,
The thermal expansion coefficient of the piezoelectric sensor and the insulating ring is smaller than the thermal expansion coefficient of the outer casing, and the thermal expansion coefficient of the rear electrode and the first inner body is larger than the thermal expansion coefficient of the outer casing. is set, the external housing and the thermal expansion amount of the internal structure is matched, pressure Kasensa characterized in that to prevent fluctuations in the preload force acting on the piezoelectric sensor. A diaphragm is formed integrally with the front electrode and sealed at the front end of the outer casing,
The pressure sensor according to claim 1 or 2, wherein the piezoelectric sensor is disposed between a front electrode and a rear electrode provided at a rear portion of the diaphragm . The pressure sensor according to any one of claims 1 to 3, wherein the piezoelectric sensor is formed by laminating a plurality of piezoelectric elements made of a Langasite crystal using a horizontal axis effect .

Images